Therefore when 64 grammes of calcium carbide are decomposed by water, or when 18 grammes of water are decomposed by calcium carbide (the by- product in each case being calcium hydroxide or slaked lime, for the formation of which a further 18 grammes of water must be present in the second instance), 29.1 large calories are set free. It is not possible yet to determine thermo-chemical data with extreme accuracy, especially on such a material as calcium carbide, which is hardly to be procured in a state of chemical purity; and so the value 28.454 calories experimentally found by Lewes agrees very satisfactorily, considering all things, with the calculated value 29.1 calories. It is to be noticed, however, that the above calculated value has been deduced on the assumption that the calcium hydroxide is obtained as a dry powder; but as slaked lime is somewhat soluble in water, and as it evolves 3 calories in so dissolving, if sufficient water is present to take up the calcium hydroxide entirely into the liquid form (i.e., that of a solution), the amount of heat set free will be greater by those 3 calories, i.e., 32.1 large calories altogether.

THE PROCESS OF GENERATION.--Taking 28 as the number of large calories developed when 64 grammes of ordinary commercial calcium carbide are decomposed with sufficient water to leave dry solid calcium hydroxide as the by-product in acetylene generation, this quantity of heat is capable of exerting any of the following effects. It is sufficient (1) to raise 1000 grammes of water through 28° C., say from 10° C. (50° F., which is roughly the temperature of ordinary cold water) to 38° C. It is sufficient (2) to raise 64 grammes of water (a weight equal to that of the carbide decomposed) through 438° C., if that were possible. It would raise (3) 311 grammes of water through 90° C., i.e., from 10° C. to the boiling-point. If, however, instead of remaining in the liquid state, the water were converted into vapour, the same quantity of heat would suffice (4) to change 44.7 grammes of water at 10° C. into steam at 100° C.; or (5) to change 46.7 grammes of water at 10° C. into vapour at the same temperature. It is an action of the last character which takes place in acetylene generators of the most modern and usual pattern, some of the surplus water being evaporated and carried away as vapour at a comparatively low temperature with the escaping gas; for it must be remembered that although steam, as such, condenses into liquid water immediately the surrounding temperature falls below 100° C., the vapour of water remains uncondensed, even at temperatures below the freezing- point, when that vapour is distributed among some permanent gas--the precise quantity of vapour so remaining being a function of the temperature and barometric height. Thus it appears that if the heat evolved during the decomposition of calcium carbide is not otherwise consumed, it is sufficient in amount to vaporise almost exactly 3 parts by weight of water for every 4 parts of carbide attacked; but if it were expended upon some substance such as acetylene, calcium carbide, or steel, which, unlike water, could not absorb an extra amount by changing its physical state (from solid to liquid, or from liquid to gas), the heat generated during the decomposition of a given weight of carbide would suffice to raise an equal weight of the particular substance under consideration to a temperature vastly exceeding 438° C. The temperature attained, indeed, measured in Centigrade degrees, would be 438 multiplied by the quotient obtained on dividing the specific heat of water by the specific heat of the substance considered: which quotient, obviously, is the "reciprocal" of the specific heat of the said substance.

The analogy to the combustion of coal mentioned on a previous page shows that although the quantity of heat evolved during a certain chemical reaction is strictly fixed, the temperature attained is dependent on the time over which the reaction is spread, being higher as the process is more rapid. This is due to the fact that throughout the whole period of reaction heat is escaping from the mass, and passing into the atmosphere at a fairly constant speed; so that, clearly, the more slowly heat is produced, the better opportunity has it to pass away, and the less of it is left to collect in the material under consideration. During the action of an acetylene generator, there is a current of gas constantly travelling away from the carbide, there is vapour of water constantly escaping with the gas, there are the walls of the generator itself constantly exposed to the cooling action of the atmosphere, and there is either a mass of calcium carbide or of water within the generator. It is essential for good working that the temperature of both the acetylene and the carbide shall be prevented from rising to any noteworthy extent; while the amount of heat capable of being dissipated into the air through the walls of the apparatus in a given time is narrowly limited, depending upon the size and shape of the generator, and the temperature of the surrounding air. If, then, a small, suitably designed generator is working quite slowly, the loss of heat through the external walls of the apparatus may easily be rapid enough to prevent the internal temperature from rising objectionably high; but the larger the generator, and the more rapidly it is evolving gas, the less does this become possible. Since of the substances in or about a generator water is the one which has by far the largest capacity for absorbing heat, and since it is the only substance to which any necessary quantity of heat can be safely or conveniently transmitted, it follows that the larger in size an acetylene generator is, or the more rapidly that generator is made to deliver gas, the more desirable is it to use water as the means for dissipating the surplus heat, and the more necessary is it to employ an apparatus in which water is in large chemical excess at the actual place of decomposition.

The argument is sometimes advanced that an acetylene generator containing carbide in excess will work satisfactorily without exhibiting an undesirable rise in internal temperature, if the vessel holding the carbide is merely surrounded by a large quantity of cold water. The idea is that the heat evolved in that particular portion of the charge which is suffering decomposition will be communicated with sufficient speed throughout the whole mass of calcium carbide present, whence it will pass through the walls of the containing vessel into the water all round. Provided the generator is quite small, provided the carbide container is so constructed as to possess the maximum of superficial area with the minimum of cubical capacity (a geometrical form to which the sphere, and in one direction the cylinder, are diametrically opposed), and provided the walls of the container do not become coated internally or externally with a coating of lime or water scale so as to diminish in heat- transmitting power, an apparatus designed in the manner indicated is undoubtedly free from grave objection; but immediately any of those provisions is neglected, trouble is likely to ensue, for the heat will not disappear from the place of actual reaction at the necessary speed. Apparent proof that heat is not accumulating unduly in a water-jacketed carbide container even when the generator is evolving gas at a fair speed is easy to obtain; for if, as usually happens, the end of the container through which the carbide is inserted is exposed to the air, the hand may be placed upon it, and it will be found to be only slightly warm to the touch. Such a test, however, is inconclusive, and frequently misleading, because if more than a pound or two of carbide is present as an undivided mass, and if water is allowed to attack one portion of it, that particular portion may attain a high temperature while the rest is comparatively cool: and if the bulk of the carbide is comparatively cool, naturally the walls of the containing vessel themselves remain practically unheated. Three causes work together to prevent this heat being dissipated through the walls of the carbide vessel with sufficient rapidity. In the first place, calcium carbide itself is a very bad conductor of heat. So deficient in heat-conducting power is it that a lump a few inches in diameter may be raised to redness in a gas flame at one spot, and kept hot for some minutes, while the rest of the mass remains sufficiently cool to be held comfortably in the fingers. In the second place, commercial carbide exists in masses of highly irregular shape, so that when they are packed into any vessel they only touch at their angles and edges; and accordingly, even if the material were a fairly good heat conductor of itself, the air or gas present between each lump would act as an insulator, protecting the second piece from the heat generated in the first. In the third place, the calcium hydroxide produced as the by-product when calcium carbide is decomposed by water occupies considerably more space than the original carbide--usually two or three times as much space, the exact figures depending upon the conditions in which it is formed--and therefore a carbide container cannot advisedly be charged with more than one-third the quantity of solid which it is apparently capable of holding. The remaining two-thirds of the space is naturally full of air when the container is first put into the generator, but the air is displaced by acetylene as soon as gas production begins. Whether that space, however, is occupied by air, by acetylene, or by a gradually growing loose mass of slaked lime, each separate lump of hot carbide is isolated from its neighbours by a material which is also a very bad heat conductor; and the heat has but little opportunity of distributing itself evenly. Moreover, although iron or steel is a notably better conductor of heat than any of the other substances present in the carbide vessel, it is, as a metal, only a poor conductor, being considerably inferior in this respect to copper. If heat dissipation were the only point to be studied in the construction of an acetylene apparatus, far better results might be obtained by the employment of copper for the walls of the carbide container; and possibly in that case a generator of considerable size, fitted with a water- jacketed decomposing vessel, might be free from the trouble of overheating. Nevertheless it will be seen in Chapter VI. that the use of copper is not permissible for such purposes, its advantages as a good conductor of heat being neutralised by its more important defects.

When suitable precautions are not taken to remove the heat liberated in an acetylene apparatus, the temperature of the calcium carbide occasionally rises to a remarkable degree. Investigating this point, Caro has studied the phenomena of heat production in a "dipping" generator-- i.e., an apparatus in which a cage of carbide is alternately immersed in and lifted out of a vessel containing water. Using a generator designed to supply five burners, he has found a maximum recording thermometer placed in the gas space of the apparatus to give readings generally between 60° and 100° C.; but in two tests out of ten he obtained temperatures of about 160° C. To determine the actual temperature of the calcium carbide itself, he scattered amongst the carbide charge fragments of different fusible metallic alloys which were known to melt or soften at certain different temperatures. In all his ten tests the alloys melting at 120° C. were fused completely; in two tests other alloys melting at 216° and 240° C. showed signs of fusion; and in one test an alloy melting at 280° C. began to soften. Working with an experimental apparatus constructed on the "dripping" principle-- i.e., a generator in which water is allowed to fall in single drops or as a fine stream upon a mass of carbide--with the deliberate object of ascertaining the highest temperatures capable of production when calcium carbide is decomposed in this particular fashion, and employing for the measurement of the heat a Le Chatelier thermo-couple, with its sensitive wires lying among the carbide lumps, Lewes has observed a maximum temperature of 674° C. to be reached in 19 minutes when water was dripped upon 227 grammes of carbide at a speed of about 8 grammes per minute. In other experiments he used a laboratory apparatus designed upon the "dipping" principle, and found maximum temperatures, in four different trials, of 703°, 734°, 754°, and 807° C., which were reached in periods of time ranging from 12 to 17 minutes. Even allowing for the greater delicacy of the instrument adopted by Lewes for measuring the temperature in comparison with the device employed by Caro, there still remains an astonishing difference between Caro's maximum of 280° and Lewes' maximum of 807° C. The explanation of this discrepancy is to be inferred from what has just been said. The generator used by Caro was properly made of metal, was quite small in size, was properly designed with some skill to prevent overheating as much as possible, and was worked at the speed for which it was intended--in a word, it was as good an apparatus as could be made of this particular type. Lewes' generator was simply a piece of glass and metal, in which provisions to avoid overheating were absent; and therefore the wide difference between the temperatures noted does not suggest any inaccuracy of observation or experiment, but shows what can be done to assist in the dissipation of heat by careful arrangement of parts. The difference in temperature between the acetylene and the carbide in Caro's test accentuates the difficulty of gauging the heat in a carbide vessel by mere external touch, and supplies experimental proof of the previous assertions as to the low heat-conducting power of calcium carbide and of the gases of the decomposing vessel. It must not be supposed that temperatures such as Lewes has found ever occur in any commercial generator of reasonably good design and careful construction; they must be regarded rather as indications of what may happen in an acetylene apparatus when the phenomena accompanying the evolution of gas are not understood by the maker, and when all the precautions which can easily be taken to avoid excessive heating have been omitted, either by building a generator with carbide in excess too large in size, or by working it too rapidly, or more generally by adopting a system of construction unsuited to the ends in view. The fact, however, that Lewes has noted the production of a temperature of 807° C. is important; because this figure is appreciably above the point 780° C., at which acetylene decomposes into its elements in the absence of air.

Nevertheless the production of a temperature somewhat exceeding 100° C. among the lumps of carbide actually undergoing decomposition can hardly be avoided in any practical generator. Based on a suggestion in the "Report of the Committee on Acetylene Generators" which was issued by the British Home Office in 1902, Fouché has proposed that 130° C., as measured with the aid of fusible metallic rods, [Footnote: An alloy made by melting together 55 parts by weight of commercial bismuth and 45 parts of lead fuses at 127° C., and should be useful in performing the tests.] should be considered the maximum permissible temperature in any part of a generator working at full speed for a prolonged period of time. Fouché adopts this figure on the ground that 130° C. sensibly corresponds with the temperature at which a yellow substance is formed in a generator by a process of polymerisation; and, referring to French conditions, states that few actual apparatus permit the development of so high a temperature. As a matter of fact, however, a fairly high temperature among the carbide is less important than in the gas, and perhaps it would be better to say that the temperature in any part of a generator occupied by acetylene should not exceed 100° C. Fraenkel has carried out some experiments upon the temperature of the acetylene immediately after evolution in a water-to-carbide apparatus containing the carbide in a subdivided receptacle, using an apparatus now frequently described as belonging to the "drawer" system of construction. When a quantity of about 7 lb. of carbide was distributed between 7 different cells of the receptacle, each cell of which had a capacity of 25 fluid oz., and the apparatus was caused to develop acetylene at the rate of 7 cubic feet per hour, maximum thermometers placed immediately over the carbide in the different cells gave readings of from 70° to 90° C., the average maximum temperature being about 80° C. Hence the Austrian code of rules issued in 1905 governing the construction of acetylene apparatus contains a clause to the effect that the temperature in the gas space of a generator must never exceed 80° C.; whereas the corresponding Italian code contains a similar stipulation, but quotes the maximum temperature as 100° C. (vide Chapter IV.).

It is now necessary to see why the production of an excessively high temperature in an acetylene generator has to be avoided. It must be avoided, because whenever the temperature in the immediate neighbourhood of a mass of calcium carbide which is evolving acetylene under the attack of water rises materially above the boiling-point of water, one or more of three several objectionable effects is produced--(a) upon the gas generated, (b) upon the carbide decomposed, and (c) upon the general chemical reaction taking place.

It has been stated above that in moat generators when the action between the carbide and the water is proceeding smoothly, it occurs according to equation (2)--

(2) CaC_2 + 2H_2O = C_2H_2 + Ca(OH)_2

rather than in accordance with equation (1)--

(1) CaC_2 + H_2O = C_2H_2 + CaO.

This is because calcium oxide, or quicklime, the by-product in (1), has considerable affinity for water, evolving a noteworthy quantity of heat when it combines with one molecule of water to form one molecule of calcium hydroxide, or slaked lime, the by-product in (2). If, then, a small amount of water is added to a large amount of calcium carbide, the corresponding quantity of acetylene may be liberated on the lines of equation (1), and there will remain behind a mixture of unaltered calcium carbide, together with a certain amount of calcium oxide. Inasmuch as both these substances possess an affinity for water (setting heat free when they combine with it), when a further limited amount of water is introduced into the mixture some of it will probably be attracted to the oxide instead of to the carbide present. It is well known that at ordinary temperatures quicklime absorbs moisture, or combines with water, to produce slaked lime; but it is equally well known that in a furnace, at about a red heat, slaked lime gives up water and changes into quicklime. The reaction, in fact, between calcium oxide and water is reversible, and whether those substances combine or dissociate is simply a question of temperature. In other words, as the temperature rises, the heat of hydration of calcium oxide diminishes, and calcium hydroxide becomes constantly a less stable material. If now it should happen that the affinity between calcium carbide and water should not diminish, or should diminish in a lower ratio than the affinity between calcium oxide and water as the temperature of the mass rises from one cause or other, it is conceivable that at a certain temperature calcium carbide might be capable of withdrawing the water of hydration from the molecule of slaked lime, converting the latter into quicklime, and liberating one molecule of acetylene, thus--

(3) CaC_2 + Ca_2(OH) = C_2H_2 + 2CaO.

It has been proved that a reaction of this character does occur, the temperature necessary to determine it being given by Lewes as from 420° to 430° C., which is not much more than half that which he found in a generator having carbide in excess, albeit one of extremely bad design. Treating this reaction in the manner previously adopted, the thermo- chemical phenomena of equation (3) are:

[1 Footnote: Into its elements, Ca, O_2, and H_2; cf. footnote, p: 31.]

Or, since the calcium hydroxide is only dehydrated without being entirely decomposed, and only one molecule of water is broken up, it may be written:

which comes to the same thing. Putting the matter in another shape, it may be said that the reaction between calcium carbide and water is exothermic, evolving either 14.0 or 29.1 calories according as the byproduct is calcium oxide or solid calcium hydroxide; and therefore either reaction proceeds without external assistance in the cold. The reaction between carbide and slaked lime, however, is endothermic, absorbing 1.1 calories; and therefore it requires external assistance (presence of an elevated temperature) to start it, or continuous introduction of heat (as from the reaction between the rest of the carbide present and the water) to cause it to proceed. Of itself, and were it not for the disadvantages attending the production of a temperature remotely approaching 400° C. in an acetylene generator, which disadvantages will be explained in the following paragraphs, there is no particular reason why reaction (3) should not be permitted to occur, for it involves (theoretically) no loss of acetylene, and no waste of calcium carbide. Only one specific feature of the reaction has to be remembered, and due practical allowance made for it. The reaction represented by equation (2) proceeds almost instantaneously when the calcium carbide is of ordinarily good quality, and the acetylene resulting therefrom is wholly generated within a very few minutes. Equation (3), on the contrary, consumes much time for its completion, and the gas corresponding with it is evolved at a gradually diminishing speed which may cause the reaction to continue for hours--a circumstance that may be highly inconvenient or quite immaterial according to the design of the apparatus. When, however, it is desired to construct an automatic acetylene generator, i.e., an apparatus in which the quantity of gas liberated has to be controlled to suit the requirements of any indefinite number of burners in use on different occasions, equation (3) becomes a very important factor in the case. To determine the normal reaction (No. 2) of an acetylene generator, 64 parts by weight of calcium carbide must react with 36 parts of water to yield 26 parts by weight of acetylene, and apparently both carbide and water are entirely consumed; but if opportunity is given for the occurrence of reaction (3), another 64 parts by weight of carbide may be attacked, without the addition of any more water, producing, inevitably, another 26 parts of acetylene. If, then, water is in chemical excess in the generator, all the calcium carbide present will be decomposed according to equation (2), and the action will take place without delay; after a few minutes' interval the whole of the acetylene capable of liberation will have been evolved, and nothing further can possibly happen until another charge of carbide is inserted in the apparatus. If, on the other hand, calcium carbide is in chemical excess in the generator, all the water run in will be consumed according to equation (2), and this action will again take place without delay; but unless the temperature of the residual carbide has been kept well below 400° C., a further evolution of gas will occur which will not cease for an indeterminate period of time, and which, by strict theory, given the necessary conditions, might continue until a second volume of acetylene equal to that liberated at first had been set free. In practice this phenomenon of a secondary production of gas, which is known as "after-generation," is regularly met with in all generators where the carbide is in excess of the water added; but the amount of acetylene so evolved rarely exceeds one-quarter or one-third of the main make. The actual amount evolved and the rate of evolution depend, not merely upon the quantity of undecomposed carbide still remaining in contact with the damp lime, but also upon the rapidity with which carbide naturally decomposes in presence of liquid water, and the size of the lumps. Where "after-generation" is caused by the ascent of water vapour round lumps of carbide, the volume of gas produced in a given interval of time is largely governed by the temperature prevailing and the shape of the apparatus. It is evident that even copious "after-generation" is a matter of no consequence in any generator provided with a holder to store the gas, assuming that by some trustworthy device the addition of water is stopped by the time that the holder is two-thirds or three-quarters full. In the absence of a holder, or if the holder fitted is too small to serve its proper purpose, "aftergeneration" is extremely troublesome and sometimes dangerous, but a full discussion of this subject must be postponed to the next chapter.

EFFECT OF HEAT ON ACETYLENE.--The effect of excessive retention of heat in an acetylene generator upon the gas itself is very marked, as acetylene begins spontaneously to suffer change, and to be converted into other compounds at elevated temperatures. Being a purely chemical phenomenon, the behaviour of acetylene when exposed to heat will be fully discussed in Chapter VI. when the properties of the gas are being systematically dealt with. Here it will be sufficient to assume that the character of the changes taking place is understood, and only the practical results of those changes as affecting the various components of an acetylene installation have to be studied. According to Lewes, acetylene commences to "polymerise" at a temperature of about 600° C., when it is converted into other hydrocarbons having the same percentage composition, but containing more atoms of carbon and hydrogen in their molecules. The formula of acetylene is C_2H_2 which means that 2 atoms of carbon and 2 atoms of hydrogen unite to form 1 molecule of acetylene, a body evidently containing roughly 92.3 per cent. by weight of carbon and 7.7 per cent. by weight of hydrogen. One of the most noteworthy substances produced by the polymerisation of acetylene is benzene, the formula of which is C_6H_6, and this is formed in the manner indicated by the equation--

(4) 3C_2H_2 = C_6H_6.

Now benzene also contains 92.3 per cent. of carbon and 7.7 per cent. by weight of hydrogen in its composition, but its molecule contains 6 atoms of each element. When the chemical formula representing a compound body indicates a substance which is, or can be obtained as, a gas or vapour, it convoys another idea over and above those mentioned on a previous page. The formula "C_2H_2," for example, means 1 molecule, or 26 parts by weight of acetylene, just as "H_2" means 1 molecule, or 2 parts by weight of hydrogen; but both formulæ also mean equal parts by volume of the respective substances, and since H_2 must mean 2 volumes, being twice H, which is manifestly 1, C_2H_2 must mean 2 volumes of acetylene as well. Thus equation (4) states that 6 volumes of acetylene, or 3 x 26 parts by weight, unite to form 2 volumes of benzene, or 78 parts by weight. If these hydrocarbons are burnt in air, both are indifferently converted into carbon dioxide (carbonic acid gas) and water vapour; and, neglecting for the sake of simplicity the nitrogen of the atmosphere, the processes may be shown thus:

(5) 2C_2H_2 + 5O_2 = 4CO_2 + 2H_2O.

(6) 2C_6H_6 + 15O_2 = 12CO_2 + 6H_2O.

Equation (5) shows that 4 volumes of acetylene combine with 10 volumes of oxygen to produce 8 volumes of carbon dioxide and 4 of water vapour; while equation (6) indicates that 4 volumes of benzene combine with 30 volumes of oxygen to yield 24 volumes of carbon dioxide and 12 of water vapour. Two parts by volume of acetylene therefore require 5 parts by volume of oxygen for perfect combustion, whereas two parts by volume of benzene need 15--i.e., exactly three times as much. In order to work satisfactorily, and to develop the maximum of illuminating power from any kind of gas consumed, a gas-burner has to be designed with considerable skill so as to attract to the base of the flame precisely that volume of air which contains the quantity of oxygen necessary to insure complete combustion, for an excess of air in a flame is only less objectionable than a deficiency thereof. If, then, an acetylene burner is properly constructed, as most modern ones are, it draws into the flame air corresponding with two and a half volumes of oxygen for every one volume of acetylene passing from the jets; whereas if it were intended for the combustion of benzene vapour it would have to attract three times that quantity. Since any flame supplied with too little air tends to emit free carbon or soot, it follows that any well-made acetylene burner delivering a gas containing benzene vapour will yield a more or lens smoky flame according to the proportion of benzene in the acetylene. Moreover, at ordinary temperatures benzene is a liquid, for it boils at 81° C., and although, as was explained above in the case of water, it is capable of remaining in the state of vapour far below its boiling-point so long as it is suspended in a sufficiency of some permanent gas like acetylene, if the proportion of vapour in the gas at any given temperature exceeds a certain amount the excess will be precipitated in the liquid form; while as the temperature falls the proportion of vapour which can be retained in a given volume of gas also diminishes to a noteworthy extent. Should any liquid, be it water or benzene, or any other substance, separate from the acetylene under the influence of cold while the gas is passing through pipes, the liquid will run downwards to the lowest points in those pipes; and unless due precautions are taken, by the insertion of draw-off cocks, collecting wells, or the like, to withdraw the deposited water or other liquid, it will accumulate in all bends, angles, and dips till the pipes are partly or completely sealed against the passage of gas, and the lights will either "jump" or be extinguished altogether. In the specific case of an acetylene generator this trouble is very likely to arise, even when the gas is not heated sufficiently during evolution for polymerisation to occur and benzene or other liquid hydrocarbons to be formed, because any excess of water present in the decomposing vessel is liable to be vaporised by the heat of the reaction--as already stated it is desirable that water shall be so vaporised--and will remain safely vaporised as long as the pipes are kept warm inside or near the generator; but directly the pipes pass away from the hot generator the cooling action of the air begins, and some liquid water will be immediately produced. Like the phenomenon of after- generation, this equally inevitable phenomenon of water condensation will be either an inconvenience or source of positive danger, or will be a matter of no consequence whatever, simply as the whole acetylene installation, including the service-pipes, is ignorantly or intelligently built.

As long as nothing but pure polymerisation happens to the acetylene, as long, that is to say, as it is merely converted into other hydrocarbons also having the general formula C_(2n)H_(2n), no harm will be done to the gas as regards illuminating power, for benzene burns with a still more luminous flame than acetylene itself; nor will any injury result to the gas if it is required for combustion in heating or cooking stoves beyond the fact that the burners, luminous or atmospheric, will be delivering a material for the consumption of which they are not properly designed. But if the temperature should rise much above the point at which benzene is the most conspicuous product of polymerisation, other far more complicated changes occur, and harmful effects may be produced in two separate ways. Some of the new hydrocarbons formed may interact to yield a mixture of one or more other hydrocarbons containing a higher proportion of carbon than that which is present in acetylene and benzene, together with a corresponding proportion of free hydrogen; the former will probably be either liquids or solids, while the latter burns with a perfectly non-luminous flame. Thus the quantity of gas evolved from the carbide and passed into the holder is less than it should be, owing to the condensation of its non-gaseous constituents. To quote an instance of this, Haber has found 15 litres of acetylene to be reduced in volume to 10 litres when the gas was heated to 638° C. By other changes, some "saturated hydrocarbons," i.e., bodies having the general formula C_nH_(2n+2), of which methane or marsh-gas, CH_4 is the best known, may be produced; and those all possess lower illuminating powers than acetylene. In two of those experiments already described, where Lewes observed maximum temperatures ranging from 703° to 807° C., samples of the gas which issued when the heat was greatest were submitted to chemical analysis, and their illuminating powers were determined. The figures he gives are as follows:

The average illuminating power of these mixed gases is about 126 candles per 5 cubic feet, whereas that of pure acetylene burnt under good laboratory conditions is 240 candles per 5 cubic feet. The product, it will be seen, had lost almost exactly 50 per cent. of its value as an illuminant, owing to the excessive heating to which it had been, exposed. Some of the liquid hydrocarbons formed at the same time are not limpid fluids like benzene, which is less viscous than water, but are thick oily substances, or even tars. They therefore tend to block the tubes of the apparatus with great persistence, while the tar adheres to the calcium carbide and causes its further attack by water to be very irregular, or even altogether impossible. In some of the very badly designed generators of a few years back this tarry matter was distinctly visible when the apparatus was disconnected for recharging, for the spent carbide was exceptionally yellow, brown, or blackish in colour, [Footnote: As will be pointed out later, the colour of the spent lime cannot always be employed as a means for judging whether overheating has occurred in a generator.] and the odour of tar was as noticeable as that of crude acetylene.

There is another effect of heat upon acetylene, more calculated to be dangerous than any of those just mentioned, which must not be lost sight of. Being an endothermic substance, acetylene is prone to decompose into its elements--

(7) C_2H_2 -> C_2 + H_2

whenever it has the opportunity; and the opportunity arrives if the temperature of the gas risen to 780° C., or if the pressure under which the gas is stored exceeds two atmospheres absolute (roughly 30 lb. per square inch). It decomposes, be it carefully understood, in the complete absence of air, directly the smallest spark of red-hot material or of electricity, or directly a gentle shock, such as that of a fall or blow on the vessel holding it, is applied to any volume of acetylene existing at a temperature exceeding 780° or at a gross pressure of 30 lb. per square inch; and however large that volume may be, unless it is contained in tubes of very small diameter, as will appear hereafter, the decomposition or dissociation into its elements will extend throughout the whole of the gas. Equation (7) states that 2 volumes of acetylene yield 2 volumes of hydrogen and a quantity of carbon which would measure 2 volumes were it obtained in the state of gas, but which, being a solid, occupies a space that may be neglected. Apparently, therefore, the dissociation of acetylene involves no alteration in volume, and should not exhibit explosive effects. This is erroneous, because 2 volumes of acetylene only yield exactly 2 volumes of hydrogen when both gases are measured at the same temperature, and all gases increase in volume as their temperature rises. As acetylene is endothermic and evolves much heat on decomposition, and as that heat must primarily be communicated to the hydrogen, it follows that the latter must be much hotter than the original acetylene; the hydrogen accordingly strives to fill a much larger space than that occupied by the undecomposed gas, and if that gas is contained in a closed vessel, considerable internal pressure will be set up, which may or may not cause the vessel to burst.

What has been said in the preceding paragraph about the temperature at which acetylene decomposes is only true when the gas is free from any notable quantity of air. In presence of air, acetylene inflames at a much lower temperature, viz., 480° C. In a manner precisely similar to that of all other combustible gases, if a stream of acetylene issues into the atmosphere, as from the orifices of a burner, the gas catches fire and burns quietly directly any substance having a temperature of 480° C. or upwards is brought near it; but if acetylene in bulk is mixed with the necessary quantity of air to support combustion, and any object exceeding 480° C. in temperature comes in contact with it, the oxidation of the hydrocarbon proceeds at such a high rate of speed as to be termed an explosion. The proportion of air needed to support combustion varies with every combustible material within known limits (cf. Chapter VI.), and according to Eitner the smallest quantity of air required to make acetylene burn or explode, as the case may be, is 25 per cent. If, by ignorant design or by careless manipulation, the first batches of acetylene evolved from a freshly charged generator should contain more than 25 per cent. of air; or if in the inauguration of a new installation the air should not be swept out of the pipes, and the first makes of gas should become diluted with 25 to 50 per cent. of air, any glowing body whose temperature exceeds 480° C. will fire the gas; and, as in the former instance, the flame will extend all through the mass of acetylene with disastrous violence and at enormous speed unless the gas is stored in narrow pipes of extremely small diameter. Three practical lessons are to be learnt from this circumstance: first, tobacco-smoking must never be permitted in any building where an escape of raw acetylene is possible, because the temperature of a lighted cigar, &c., exceeds 480° C.; secondly, a light must never be applied to a pipe delivering acetylene until a proper acetylene burner has been screwed into the aperture; thirdly, if any appreciable amount of acetylene is present in the air, no operation should be performed upon any portion of an acetylene plant which involves such processes as scraping or chipping with the aid of a steel tool or shovel. If, for example, the iron or stoneware sludge-pipe is choked, or the interior of the dismantled generator is blocked, and attempts are made to remove the obstruction with a hard steel tool, a spark is very likely to be formed which, granting the existence of sufficient acetylene in the air, is perfectly able to fire the gas. For all such purposes wooden implements only are best employed; but the remark does not apply to the hand-charging of a carbide-to-water generator through its proper shoot. Before passing to another subject, it may be remarked that a quantity of air far less than that which causes acetylene to become dangerous is objectionable, as its presence is apt to reduce the illuminating power of the gas unduly.

EFFECT OF HEAT ON CARBIDE.--Chemically speaking, no amount of heat possible of attainment in the worst acetylene generator can affect calcium carbide in the slightest degree, because that substance may be raised to almost any temperature short of those distinguishing the electric furnace, without suffering any change or deterioration. In the absence of water, calcium carbide is as inert a substance as can well be imagined: it cannot be made to catch fire, for it is absolutely incombustible, and it can be heated in any ordinary flame for reasonable periods of time, or thrown into any non-electrical furnace without suffering in the least. But in presence of water, or of any liquid containing water, matters are different. If the temperature of an acetylene generator rises to such an extent that part of the gas is polymerised into tar, that tar naturally tends to coat the residual carbide lumps, and, being greasy in character, more or less completely protects the interior from further attack. Action of this nature not only means that the acetylene is diminished in quantity and quality by partial decomposition, but it also means that the make is smaller owing to imperfect decomposition of the carbide: while over and above this is the liability to nuisance or danger when a mass of solid residue containing some unaltered calcium carbide is removed from the apparatus and thrown away. In fact, whenever the residue of a generator is not so saturated with excess of water as to be of a creamy consistency, it should be put into an uncovered vessel in the open air, and treated with some ten times its volume of water before being run into any drain or closed pipe where an accumulation of acetylene may occur. Even at temperatures far below those needed to determine a production of tar or an oily coating on the carbide, if water attacks an excess of calcium carbide somewhat rapidly, there is a marked tendency for the carbide to be "baked" by the heat produced; the slaked lime adhering to the lumps as a hard skin which greatly retards the penetration of more water to the interior.

COLOUR OF SPENT CARBIDE.--In the early days of the industry, it was frequently taken for granted that any degradation in the colour of the spent lime left in an acetylene generator was proof that overheating had taken place during the decomposition of the carbide. Since both calcium oxide and hydroxide are white substances, it was thought that a brownish, greyish, or blackish residue must necessarily point to incipient polymerisation of the gas. This view would be correct if calcium carbide were prepared in a state of chemical purity, for it also is a white body. Commercial carbide, however, is not pure; it usually contains some foreign matter which tints the residue remaining after gasification. When a manufacturer strives to give his carbide the highest gas-making power possible he frequently increases the proportion of carbon in the charge submitted to electric smelting, until a small excess is reached, which remains in the free state amongst the finished carbide. After decomposition the fine particles of carbon stain the moist lime a bluish grey tint, the depth of shade manifestly depending upon the amount present. If such a sludge is copiously diluted with water, particles of carbon having the appearance and gritty or flaky nature of coke often rise to the surface or fall to the bottom of the liquid; whence they can easily be picked out and identified as pure or impure carbon by simple tests. Similarly the lime or carbon put into the electric furnace may contain small quantities of compounds which are naturally coloured; and which, reappearing in the sludge either in their original or in a different state of combination, confer upon the sludge their characteristic tinge. Spent lime of a yellowish brown colour is frequently to be met with in circumstances that are clearly no reproach to the generator. Doubtless the tint is due to the presence of some coloured metallic oxide or other compound which has escaped reduction in the electric furnace. The colour which the residual lime afterwards assumes may not be noticeable in the dry carbide before decomposition, either because some change in the colour-giving impurity takes place during the chemical reactions in the generator or because the tint is simply masked by the greyish white of the carbide and its free carbon. Hence it follows that a bad colour in the waste lime removed from a generator only points to overheating and polymerisation of the acetylene when corroborative evidence is obtained--such as a distinct tarry smell, the actual discovery of oily or tarry matters elsewhere, or a grave reduction in the illuminating power of the gas.

MAXIMUM ATTAINABLE TEMPERATURES.--In order to discover the maximum temperature which can be reached in or about an acetylene generator when an apparatus belonging to one of the best types is fed at a proper rate with calcium carbide in lumps of the most suitable size, the following calculation may be made. In the first place, it will be assumed that no loss of heat by radiation occurs from the walls of the generator; secondly, the small quantity of heat taken up by the calcium hydroxide produced will be ignored; and, thirdly, the specific heat of acetylene will be assumed to be 0.25, which is about its most probable value. Now, a hand-fed carbide-to-water generator will work with half a gallon of water for every 1 lb. of carbide decomposed, quantities which correspond with 320 grammes of water per 64 grammes (1 molecular weight) of carbide. Of those 320 grammes of water, 18 are chemically destroyed, leaving 302. The decomposition of 64 grammes of commercial carbide evolves 28 large calories of heat. Assuming all the heat to be absorbed by the water, 28 calories would raise 302 grammes through (28 X 1000 / 302) = 93° C., i.e., from 44.6° F. to the boiling-point. Assuming all the heat to be communicated to the acetylene, those 28 calories would raise the 26 grammes of gas liberated through (28 X 1000 / 26 / 0.25) = 4308° C., if that were possible. But if, as would actually be the case, the heat were distributed uniformly amongst the 302 grammes of water and the 20 grammes of acetylene, both gas and water would be raised through the same number of degrees, viz., 90.8° C. [Footnote: Let x = the number of large calories absorbed by the water; then 28 - x = those taken up by the gas. Then--

1000x / 302 = 1000 (28 - x) / (26 X 0.25)

whence x = 27.41; and 28 - x = 0.59.

Therefore, for water, the rise in temperature is--

27.41 X 1000 / 302 = 90.8° C.;

and for acetylene the rise is--

0.59 X 1000 / 26 / 0.25 = 90.8° C.]

If the generator were designed on lines to satisfy the United States Fire Underwriters, it would contain 8.33 lb. of water to every 1 lb. of carbide attacked; identical calculations then showing that the original temperature of the water and gas would be raised through 53.7° C. Provided the carbide is not charged into such an apparatus in lumps of too large a size, nor at too high a rate, there will be no appreciable amount of local overheating developed; and nowhere, therefore, will the rise in temperature exceed 91° in the first instance, or 54° C. in the second. Indeed it will be considerably smaller than this, because a large proportion of the heat evolved will be lost by radiation through the generator walls, while another portion will be converted from sensible into latent heat by causing part of the water to pass off as vapour with the acetylene.

EFFECT OF HIGH TEMPERATURES ON GENERATORS.--As the temperature amongst the carbide in any generator in which water is not present in large excess may easily reach 200° C. or upwards, no material ought to be employed in the construction of such generators which is not competent to withstand a considerable amount of heat in perfect safety. The ordinary varieties of soft solder applied with the bitt in all kinds of light metal-work usually melt, according to their composition, at about 180° C.; and therefore this method of making joints is only suitable for objects that are never raised appreciably in temperature above the boiling-point of water. No joint in an acetylene generator, the partial or complete failure of which would radically affect the behaviour of the apparatus, by permitting the charges of carbide and of water to come into contact at an abnormal rate of speed, by allowing the acetylene to escape directly through the crack into the atmosphere, or by enabling the water to run out of the seal of any vessel containing gas so as to set up a free communication between that vessel and the air, ought ever to be made of soft solder--every joint of this character should be constructed either by riveting, by bolting, or by doubly folding the metal sheets. Apparently, a joint constantly immersed in water on one side cannot rise in temperature above the boiling-point of the liquid, even when its other side is heated strongly; but since, even if a generator is not charged with naturally hard water, its fluid contents soon become "hard" by dissolution of lime, there is always a liability to the deposition of water scale over the joint. Such water scale is a very bad heat conductor, as is seen in steam boilers, so that a seam coated with an exceedingly thin layer of scale, and heated sharply on one side, will rise above the boiling-point of water even if the liquid on its opposite side is ice-cold. For a while the film of scale may be quite water-tight, but after it has been heated by contact with the hot metal several times it becomes brittle and cracks without warning. But there is a more important reason for avoiding the use of plumbers' solder. It might seem that as the natural hard, protective skin of the metal is liable to be injured or removed by the bending or by the drilling or punching which precedes the insertion of the rivets or studs, an application of soft solder to such a joint should be advantageous. This is not true because of the influence of galvanic action. As all soft solders consist largely of lead, if a joint is soldered, a "galvanic couple" of lead and iron, or of lead and zinc (when the apparatus is built of galvanised steel), is exposed to the liquid bathing it; and since in both cases the lead is highly electro-negative to the iron or zinc, it is the iron or zinc which suffers attack, assuming the liquid to possess any corrosive properties whatever. Galvanised iron which has been injured during the joint-making presents a zinc-iron couple to the water, but the zinc protects the iron; if a lead solder is present, the iron will begin to corrode immediately the zinc has disappeared. In the absence of lead it is the less important metal, but in the presence of lead it is the more important (the foundation) metal which is the soluble element of the couple. Where practicable, joints in an acetylene generator may safely be made by welding or by autogenous soldering ("burning"), because no other metal is introduced into the system; any other process, except that of riveting or folding, only hastens destruction of the plant. The ideal method of making joints about an acetylene generator is manifestly that of autogenous soldering, because, as will appear in Chapter IX. of this book, the most convenient and efficient apparatus for performing the operation is the oxy-acetylene blow-pipe, which can be employed so as to convert two separate pieces of similar metal into one homogeneous whole.

In less critical situations in an acetylene plant, such as the partitions of a carbide container, &c., where the collapse of the seam or joint would not be followed by any of the effects previously suggested, there is less cause for prohibiting the use of unfortified solder; but even here, two or three rivets, just sufficient to hold the metal in position if the solder should give way, are advisedly put into all apparatus. In other portions of an acetylene installation where a merely soldered joint is exposed to warm damp gas which is in process of cooling, instead of being bathed in hard water, an equal, though totally dissimilar, danger is courted. The main constituent of such solders that are capable of being applied with the bitt is lead; lead is distinctly soluble in soft or pure water; and the water which separates by condensation out of a warm damp gas is absolutely soft, for it has been distilled. If condensation takes place at or near a soldered joint in such a way that water trickles over the solder, by slow degrees the metallic lead will be dissolved and removed, and eventually a time will come when the joint is no longer tight to gas. In fact, if an acetylene installation is of more than very small dimensions, e.g., when it is intended to supply any building as large as, or larger than, the average country residence, if it is to give satisfaction to both constructor and purchaser by being quite trustworthy and, possessed of a due lease of life, say ten or fifteen years, it must be built of stouter materials than the light sheets which alone are suitable for manipulation with the soldering-iron or for bending in the ordinary type of metal press. Sound cast-iron, heavy sheet-metal, or light boiler-plate is the proper substance of which to construct all the important parts of a generator, and the joints in wrought metal must be riveted and caulked or soldered autogeneously as mentioned above. So built, the installation becomes much more costly to lay down than an apparatus composed of tinplate, zinc, or thin galvanised iron, but it will prove more economical in the long run. It is not too much to say that if ignorant and short-sighted makers in the earliest days of the acetylene industry had not recommended and supplied to their customers lightly built apparatus which has in many instances already begun to give trouble, to need repairs, and to fail by thorough corrosion--apparatus which frequently had nothing but cheapness in its favour--the use of the gas would have spread more rapidly than it has done, and the public would not now be hearing of partial or complete failures of acetylene installations. Each of these failures, whether accompanied by explosions and injury to persons or not, acts more powerfully to restrain a possible new customer from adopting the acetylene light, than several wholly successful plants urge him to take it up; for the average member of the public is not in a position to distinguish properly between the collapse of a certain generator owing to defective design or construction (which reflects no discredit upon the gas itself), and the failure of acetylene to show in practice those advantages that have been ascribed to it. One peculiar and noteworthy feature of acetylene, often overlooked, is that the apparatus is constructed by men who may have been accustomed to gas-making plant all their lives, and who may understand by mere habit how to superintend a chemical operation; but the same apparatus is used by persons who generally have no special acquaintance with such subjects, and who, very possibly, have not even burnt coal-gas at any period of their lives. Hence it happens that when some thoughtless action on the part of the country attendant of an acetylene apparatus is followed by an escape of gas from the generator, and by an accumulation of that gas in the house where the plant is situated, or when, in disregard of rules, he takes a naked light into the house and an explosion follows, the builder dismisses the episode as a piece of stupidity or wilful misbehaviour for which he can in nowise be held morally responsible; whereas the builder himself is to blame for designing an apparatus from which an escape of gas can be accompanied by sensible risks to property or life. However unpalatable this assertion may be, its truth cannot be controverted; because, short of criminal intention or insanity on the part of the attendant, it is in the first place a mere matter of knowledge and skill so to construct an acetylene plant that an escape of gas is extremely unlikely, even when the apparatus is opened for recharging, or when it is manipulated wrongly; and in the second place, it is easy so to arrange the plant that any disturbance of its functions which may occur shall be followed by an immediate removal of the surplus gas into a place of complete safety outside and above the generator-house.

GENERATION AT LOW TEMPERATURES.--In all that has been said hitherto about the reaction between calcium carbide and water being instantaneous, it has been assumed that the two substances are brought together at or about the usual temperature of an occupied room, i.e., 15 degrees C. If, however, the temperature is materially lower than this, the speed of the reaction falls off, until at -5 degrees C., supposing the water still to remain liquid, evolution of acetylene practically ceases. Even at the freezing-point of pure water gas is produced but slowly; and if a lump of carbide is thrown on to a block of ice, decomposition proceeds so gently that the liberated acetylene may be ignited to form a kind of torch, while heat is generated with insufficient rapidity to cause the carbide to sink into the block. This fact has very important bearings upon the manipulation of an acetylene generator in winter time. It is evident that unless precautions are taken those portions of an apparatus which contain water are liable to freeze on a cold night; because, even if the generator has been at work producing gas (and consequently evolving heat) till late in the evening, the surplus heat stored in the plant may escape into the atmosphere long before more acetylene has to be made, and obviously while frost is still reigning in the neighbourhood. If the water freezes in the water store, in the pipes leading therefrom, in the holder seal, or in the actual decomposing chamber, a fresh batch of gas is either totally incapable of production, because the water cannot be brought into contact with the calcium carbide in the apparatus, or it can only be generated with excessive slowness because the carbide introduced falls on to solid ice. Theoretically, too, there is a possibility that some portion of the apparatus--a pipe in particular--may be burst by the freezing, owing to the irresistible force with which water expands when it changes into the solid condition. Probably this last contingency, clearly accompanied as it would be by grave risk, is somewhat remote, all the plant being constructed of elastic material; but in practice even a simple interference with the functions of a generator by freezing, ideally of no special moment, is highly dangerous, because of the great likelihood that hurried and wholly improper attempts to thaw it will be made by the attendant. As it has been well known for many years that the solidifying point of water can be lowered to almost any degree below normal freezing by dissolving in it certain salts in definite proportions, one of the first methods suggested for preventing the formation of ice in an acetylene generator was to employ such a salt, using, in fact, for the decomposition of the carbide some saline solution which remains liquid below the minimum night temperature of the winter season. Such a process, however, has proved unsuitable for the purpose in view; and the explanation of that fact is found in what has just been stated: the "water" of the generator may admittedly be safely maintained in the fluid state, but from so cold a liquid acetylene will not be generated smoothly, if at all. Moreover, were it not so, a process of this character is unnecessarily expensive, although suitable salts are very cheap, for the water of the generator is constantly being consumed, [Footnote: It has already been said that most generators "consume" a much larger volume of water than the amount corresponding with the chemical reaction involved: the excess of water passing into the sludge or by- product. Thus a considerable quantity of any anti-freezing agent must be thrown aside each time the apparatus is cleaned out or its fluid contents are run off.] and as constantly needs renewal; which means that a fresh batch of salt would be required every time the apparatus was recharged, so long as frost existed or might be expected. A somewhat different condition obtains in the holder of an acetylene installation. Here, whenever the holder is a separate item in the plant, not constituting a portion of the generating apparatus, the water which forms the seal of a rising holder, or which fills half the space of a displacement holder, lasts indefinitely; and it behaves equally well, whatever its temperature may be, so long as it retains a fluid state. This matter will be discussed with greater detail at the end of Chapter III. At present the point to be insisted on is that the temperature in any constituent of an acetylene installation which contains water must not be permitted to fall to the freezing-point; while the water actually used for decomposition must be kept well above that temperature.

GENERATION AT HIGH TEMPERATURES.--At temperatures largely exceeding those of the atmosphere, the reaction between calcium carbide and water tends to become irregular; while at a red heat steam acts very slowly upon carbide, evolving a mixture of acetylene and hydrogen in place of pure acetylene. But since at pressures which do not materially exceed that of the atmosphere, water changes into vapour at 100° C., above that temperature there can be no question of a reaction between carbide and liquid water. Moreover, as has been pointed out, steam or water vapour will continue to exist as such at temperatures even as low as the freezing-point so long as the vapour is suspended among the particles of a permanent gas. Between calcium carbide and water vapour a double decomposition occurs chemically identical with that between carbide and liquid water; but the physical effect of the reaction and its practical bearings are considerably modified. The quantity of heat liberated when 30 parts by weight of steam react with 64 parts of calcium carbide should be essentially unaltered from that evolved when the reagent is in the liquid state; but the temperature likely to be attained when the speed of reaction remains the same as before will be considerably higher for two conspicuous reasons. In the first place, the specific heat of steam in is only 0.48, while that of liquid water is 1.0. Hence, the quantity of heat which is sufficient to raise the temperature of a given weight of liquid water through n thermometric degrees, will raise the temperature of the same weight of water vapour through rather more than 2 n degrees. In the second place, that relatively large quantity of heat which in the case of liquid water merely changes the liquid into a vapour, becoming "latent" or otherwise unrecognisable, and which, as already shown, forms roughly five-sixths of the total heat needed to convert cold water into steam, has no analogue if the water has previously been vaporised by other means; and therefore the whole of the heat supplied to water vapour raises its sensible temperature, as indicated by the thermometer. Thus it appears that, except for the sufficient amount of cooling that can be applied to a large vessel containing carbide by surrounding it with a water jacket, there is no way of governing its temperature satisfactorily if water vapour is allowed to act upon a mass of carbide--assuming, of course, that the reaction proceeds at any moderate speed, e.g., at a rate much above that required to supply one or two burners with gas.

The decomposition which with perfect chemical accuracy has been stated to occur quantitatively between 36 parts by weight, of water and 64 parts of calcium carbide scarcely ever takes place in so simple a fashion in an actual generator. Owing to the heat developed when carbide is in excess, about half the water is converted into vapour; and so the reaction proceeds in two stages: half the water added reacting with the carbide as a liquid, the other half, in a state of vapour, afterwards reacting similarly, [Footnote: This secondary reaction is manifestly only another variety of the phenomenon known as "after-generation" (cf. ante). After-generation is possible between calcium carbide and mechanically damp slaked lime, between carbide and damp gas, or between carbide and calcium hydroxide, as opportunity shall serve. In all cases the carbide must be in excess.] or hardly reacting at all, as the case may be. Suppose a vessel, A B, somewhat cylindrical in shape, is charged with carbide, and that water is admitted at the end called A. Suppose now (1) that the exit for gas is at the opposite end, B. As the lumps near A are attacked by half the liquid introduced, while the other half is changed into steam, a current, of acetylene and water vapour travels over the charge lying between the decomposing spot and the end B. During its passage the second half of the water, as vapour, reacts with the excess of carbide, the first make of acetylene being dried, and more gas being produced. Thus a second quantity of heat is developed, equal by theory to that previously evolved; but a second elevation in temperature, far more serious, and far less under control, than the former also occurs; and this is easily sufficient to determine some of those undesirable effects already described. Digressing for a moment, it may be admitted that the desiccation of the acetylene produced in this manner is beneficial, even necessary; but the advantages of drying the gas at this period of its treatment are outweighed by the concomitant disadvantages and by the later inevitable remoistening thereof. Suppose now (2) that both the water inlet and the gas exit of the carbide cylinder are at the same end, A. Again half the added water, as liquid, reacts with the carbide it first encounters, but the hot stream of damp gas is not permitted to travel over the rest of the lumps extending towards B: it is forced to return upon its steps, leaving B practically untouched. The gas accordingly escapes from the cylinder at A still loaded with water vapour, and for a given weight of water introduced much less acetylene is evolved than in the former case. The gas, too, needs drying somewhere else in the plant; but these defects are preferable to the apparent superiority of the first process because overheating is, or can be, more thoroughly guarded against.

PRESSURE IN GENERATORS.--Inasmuch as acetylene is prone to dissociate or decompose into its elements spontaneously whenever its pressure reaches 2 atmospheres or 30 lb. per square inch, as well as when its temperature at atmospheric pressure attains 780° C., no pressure approaching that of 2 atmospheres is permissible in the generator. A due observance of this rule, however, unlike a proper maintenance of a low temperature in an acetylene apparatus, is perfectly easy to arrange for. The only reason for having an appreciable positive pressure in any form of generating plant is that the gas may be compelled to travel through the pipes and to escape from the burner orifices; and since the plant is only installed to serve the burners, that pressure which best suits the burners must be thrown by the generator or its holder. Therefore the highest pressure it is ever requisite to employ in a generator is a pressure sufficient (a) to lift the gasholder bell, or to raise the water in a displacement holder, (b) to drive the gas through the various subsidiary items in the plant, such as washers and purifiers, (c) to overcome the friction in the service-pipes, [Footnote: This friction manifestly causes a loss of pressure, i.e., a fall in pressure, as a gas travels along a pipe; and, as will be shown in Chapter VII., it is the fall in pressure in a pipe rather than the initial pressure at which a gas enters a pipe that governs the volume of gas passing through that pipe. The proper behaviour and economic working of a burner (acetylene or other, luminous or incandescent) naturally depend upon the pressure in the pipe to which the burner is immediately attached being exactly suited to the design of that burner, and have nothing to do with the fall in pressure occurring in the delivery pipes. It is therefore necessary to keep entirely separate the ideas of proper burner pressure and of maximum desirable fall in pressure within the service due to friction.] and (d) to give at the points of combustion a pressure which is required by the particular burners adopted. In all except village or district installations, (c) may be virtually neglected. When the holder has a rising bell, (a) represents only an inch or so of water; but if a displacement holder is employed the pressure needed to work it is entirely indeterminate, being governed by the size and shape of the said holder. It will be argued in Chapter III. that a rising holder is always preferable to one constructed on the displacement principle. The pressure (d) at the burners may be taken at 4 inches of water as a maximum, the precise figure being dependent upon the kind of burners--luminous, incandescent, boiling, &c.--attached to the main. The pressure (b) also varies according to circumstances, but averages 2 or 3 inches. Thus a pressure in the generator exceeding that of the atmosphere by some 12 inches of water--i.e., by about 7 oz., or less than half a pound per square inch--is amply sufficient for every kind of installation, the less meritorious generators with displacement holders only excepted. This pressure, it should be noted, is the net or effective pressure, the pressure with which the gas raises the liquid in a water-gauge glass out of the level while the opposite end of the water column is exposed to the atmosphere. The absolute pressure in a vessel containing gas at an effective pressure of 12 inches of water is 7 oz. plus the normal, insensible pressure of the atmosphere itself--say 15-1/4 lb. per square inch. The liquid in a barometer which measures the pressure of the atmosphere stands at a height of 30 inches only, because that liquid is mercury, 13.6 times as heavy as water. Were it filled with water the barometer would stand at (30 X 13.6) = 408 inches, or 34 feet, approximately. Gas pressures are always measured in inches of water column, because expressed either as pounds per square inch or as inches of mercury, the figures would be so small as to give decimals of unwieldy length.