It is possible to calculate the amount of the stress at the point
at each side of the satellite where the stress is at its
greatest. We must assume the satellite to be a certain size and
density; we must also assume the crust of

181

Mars to be of some certain density. To fix our ideas on these
points I take the case of the present satellite Phobos. What
amount of stress will he exert upon the crust of Mars when he
approaches within, say, 40 miles of the planet's surface? We know
his size approximately—he is about 36 miles in diameter. We can
guess his density to be between four times that of water and
eight times that of water. We may assume the density of Mars'
surface to be about the same as that of our Earth's surface, that
is three times as dense as water. We now find that the greatest
stress tending to rend open the surface crust of Mars will be
between 4,000 and 8,000 pounds to the square foot according to
the density we assign to Phobos.

Will such a stress actually tear open the crust? We are not able
to answer this question with any certainty. Much will depend upon
the nature and condition of the crust. Thus, suppose that we are
here (Fig. 12) looking down upon the satellite which is moving
along slowly relatively to Mars' surface, in the direction of the
arrow. The satellite has just passed over a weak and cracked part
of the planet's crust. Here the stress has been sufficient to
start two cracks. Now you know how easy it is to tear a piece of
cloth when you go to the edge of it in order to make a beginning.
Here the stress from the satellite has got to the edge of the
crust. It is greatly concentrated just at the extremities of the
cracks. It will, unler such circumstances probably carry on the

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tear. If it does not do so this time, remember the satellite will
some hours later be coming over the same place again, and then
again for, at least, many hundreds of times. Then also we are not
limited to the assumption that the

{Fig. 12}

satellite is as small as Phobos. Suppose we consider the case of
a satellite approaching Mars which has a diameter double that of
Phobos; a diameter still much less than that of the larger class
of asteroids. Even at the distance

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of 65 miles the stress will now amount to as much as from 15 to
30 tons per square foot. It is almost certain that such a stress
repeated a comparatively few times over the same parts of the
planet's surface would so rend the crust as to set up lines along
which plutonic action would find a vent. That is, we might expect
along these lines all the phenomena of upheaval and volcanic
eruption which give rise to surface elevations.

The probable effect of a satellite of this dimension travelling
slowly relatively to the surface of Mars is, then, to leave a
very conspicuous memorial of his presence behind him. You see
from the diagram that this memorial will consist o: two parallel
lines of disturbance.

The linear character of the gravitational effects of the
satellite is due entirely to the motion of the satellite
relatively to the surface of the planet. If the satellite stood
still above the surface the gravitational stress in the crust
would, of course, be exerted radially outwards from the centre of
the satellite. It would attain at the central point beneath the
satellite its maximum vertical effect, and at some radial
distance measured outwards from this point, which distance we can
calculate, its maximum horizontal tearing effect. When the
satellite moves relatively to the planet's crust, the horizontal
tearing force acts differently according to whether it is
directed in the line of motion or at right angles to this line.

In the direction of motion we see that the satellite

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creates as it passes over the crust a wave of rarefaction or
tension as at D, followed by compression just beneath the
satellite and by a reversed direction of gravitational pull as
the satellite passes onwards. These stresses rapidly replace one
another as the satellite travels along. They are resisted by the
inertia of the crust, and are taken up by its elasticity. The
nature of this succession of alternate compressions and
rarefactions in the crust possess some resemblance to those
arising in an earthquake shock.

If we consider the effects taking place laterally to the line of
motion we see that there are no such changes in the nature of the
forces in the crust. At each passage of the satellite the
horizontal tearing stress increases to a maximum, when it is
exerted laterally, along the line passing through the horizontal
projection of the satellite and at right angles to the line of
motion, and again dies away. It is always a tearing stress,
renewed again and again.

This effect is at its maximum along two particular parallel lines
which are tangents to the circle of maximum horizontal stress and
which run parallel with the path of the satellite. The distance
separating these lines depend upon the elevation of the satellite
above the planet's surface. Such lines mark out the theoretical
axes of the "double canals" which future crustal movements will
more fully develop.

It is interesting to consider what the effect of such

185

conditions would be if they arose at the surface of our own
planet. We assume a horizontal force in the crust adequate to set
up tensile stresses of the order, say, of fifteen tons to the
square foot and these stresses to be repeated every few hours;
our world being also subject to the dynamic effects we recognise
in and beneath its crust.

It is easy to see that the areas over which the satellite exerted
its gravitational stresses must become the foci —foci of linear
form—of tectonic developments or crust movements. The relief of
stresses, from whatever cause arising, in and beneath the crust
must surely take place in these regions of disturbance and along
these linear areas. Here must become concentrated the folding
movements, which are under existing conditions brought into the
geosynclines, along with their attendant volcanic phenomena. In
the case of Mars such a concentration of tectonic events would
not, owing to the absence of extensive subaerial denudation and
great oceans, be complicated by the existence of such synclinal
accumulations as have controlled terrestrial surface development.
With the passage of time the linear features would probably
develop; the energetic substratum continually asserting its
influence along such lines of weakness. It is in the highest
degree probable that radioactivity plays no less a part in
Martian history than in terrestrial. The fact of radioactive
heating allows us to assume the thin surface crust and continued
sub-crustal energy throughout the entire period of the planet's
history.

186

How far willl these effects resemble the double canals of Mars?
In this figure and in the calculations I have given you I have
supposed the satellite engaged in marking the planet's surface
with two lines separated by about the interval separating the
wider double canals of Mars—that is about 220 miles apart. What
the distance between the lines will be, as already stated, will
depend upon the height of the satellite above the surface when it
comes upon a part of the crust in a condition to be affected by
the stresses it sets up in it. If the satellite does its work at
a point lower down above the surface the canal produced will be
narrower. The stresses, too, will then be much greater. I must
also observe that once the crust has yielded to the pulling
stress, there is great probability that in future revolutions of
the satellite a central fracture will result. For then all the
pulling force adds itself to the lifting force and tends to crush
the crust inwards on the central line beneath the satellite. It
is thus quite possible that the passage of a satellite may give
rise to triple lines. There is reason to believe that the canals
on Mars are in some cases triple.

I have spoken all along of the satellite moving slowly over the
surface of Mars. I have done so as I cannot at all pronounce so
readily on what will happen when the satellite's velocity over
the surface of Mars is very great. To account for all the lines
mapped by Lowell some of them must have been produced by
satellities moving relatively to the surface of Mars at
velocities so great

187

as three miles a second or even rather more. The stresses set up
are, in such cases, very difficult to estimate. It has not yet
been done. Parallel lines of greatest stress or impulse ought to
be formed as in the other case.

I now ask your attention to another kind of evidence that the
lines are due in some way to the motion of satellites passing
over the surface of Mars.

I may put the fresh evidence to which I refer, in this way: In
Lowell's map (P1. XXII, p. 192), and in a less degree in
Schiaparelli's map (ante p. 166), we are given the course of the
lines as fragments of incomplete curves. Now these curves might
have been anything at all. We must take them as they are,
however, when we apply them as a test of the theory that the
motion of a satellite round Mars can strike such lines. If it can
be shown that satellites revolving round Mars might strike just
such curves then we assume this as an added confirmation of the
hypothesis.

We must begin by realising what sort of curves a satellite which
disturbs the surface of a planet would leave behind it after its
demise. You might think that the satellite revolving round and
round the planet must simply describe a circle upon the spherical
surface of the planet: a "great circle" as it is called; that is
the greatest circle which can be described upon a sphere. This
great circle can, however, only be struck, as you will see, when
the planet is not turning upon its axis: a condition not likely
to be realised.

This diagram (PI. XXI) shows the surface of a globe

188

covered with the usual imaginary lines of latitude and longitude.
The orbit of a supposed satellite is shown by a line crossing the
sphere at some assumed angle with the equator. Along this line
the satellite always moves at uniform velocity, passing across
and round the back of the sphere and again across. If the sphere
is not turning on its polar axis then this satellite, which we
will suppose armed with a pencil which draws a line upon the
sphere, will strike a great circle right round the sphere. But
the sphere is rotating. And it is to be expected that at
different times in a planet's history the rate of rotation varies
very much indeed. There is reason to believe that our own day was
once only 2½ hours long, or thereabouts. After a preliminary rise
in velocity of axial rotation, due to shrinkage attending rapid
cooling, a planet as it advances in years rotates slower and
slower. This phenomenon is due to tidal influences of the sun or
of satellites. On the assumption that satellites fell into Mars
there would in his case be a further action tending to shorten
his day as time went on.

The effect of the rotation of the planet will be, of course, that
as the satellite advances with its pencil it finds the surface of
the sphere being displaced from under it. The line struck ceases
to be the great circle but wanders off in another curve—which is
in fact not a circle at all.

You will readily see how we find this curve. Suppose the sphere
to be rotating at such a speed that while the satellite is
advancing the distance _Oa_, the point _b_ on the

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sphere will be carried into the path of the satellite. The pencil
will mark this point. Similarly we find that all the points along
this full curved line are points which will just find themselves
under the satellite as it passes with its pencil. This curve is
then the track marked out by the revolving satellite. You see it
dotted round the back of the sphere to where it cuts the equator
at a certain point. The course of the curve and the point where
it cuts the equator, before proceeding on its way, entirely
depend upon the rate at which we suppose the sphere to be
rotating and the satellite to be describing the orbit. We may
call the distance measured round the planet's equator separating
the starting point of the curve from the point at which it again
meets the equator, the "span" of the curve. The span then depends
entirely upon the rate of rotation of the planet on its axis and
of the satellite in its orbit round the planet.

But the nature of events might have been somewhat different. The
satellite is, in the figure, supposed to be rotating round the
sphere in the same direction as that in which the sphere is
turning. It might have been that Mars had picked up a satellite
travelling in the opposite direction to that in which he was
turning. With the velocity of planet on its axis and of satellite
in its orbit the same as before, a different curve would have
been described. The span of the curve due to a retrograde
satellite will be greater than that due to a direct satellite.
The retrograde satellite will have a span more than half

190

way round the planet, the direct satellite will describe a curve
which will be less than half way round the planet: that is a span
due to a retrograde satellite will be more than 180 degrees,
while the span due to a direct satellite will be less than 180
degrees upon the planet's equator.

I would draw your attention to the fact that what the span will
be does not depend upon how much the orbit of the satellite is
inclined to the equator. This only decides how far the curve
marked out by the satellite will recede from the equator.

We find then, so far, that it is easy to distinguish between the
direct and the retrograde curves. The span of one is less, of the
other greater, than 180 degrees. The number of degrees which
either sort of curve subtends upon the equator entirely depends
upon the velocity of the satellite and the axial velocity of the
planet.

But of these two velocities that of the satellite may be taken as
sensibly invariable, when close enough to use his pencil. This
depends upon the law of centrifugal force, which teaches us that
the mass of the planet alone decides the velocity of a satellite
in its orbit at any fixed distance from the planet's centre. The
other velocity—that of the planet upon its axis—was, as we have
seen, not in the past what it is now. If then Mars, at various
times in his past history, picked up satellites, these satellites
will describe curves round him having different spans which will
depend upon the velocity of axial rotation of Mars at the time
and upon this only.

191

In what way now can we apply this knowledge of the curves
described by a satellite as a test of the lunar origin of the
lines on Mars?

To do this we must apply to Lowell's map. We pick out preferably,
of course, the most complete and definite curves. The chain of
canals of which Acheron and Erebus are members mark out a fairly
definite curve. We produce it by eye, preserving the curvature as
far as possible, till it cuts the equator. Reading the span on
the equator we find' it to be 255 degrees. In the first place we
say then that this curve is due to a retrograde satellite. We
also note on Lowell's map that the greatest rise of the curve is
to a point about 32 degrees north of the equator. This gives the
inclination of the satellite's orbit to the plane of Mars'
equator.

With these data we calculate the velocity which the planet must
have possessed at the time the canal was formed on the hypothesis
that the curve was indeed the work of a satellite. The final
question now remains If we determine the curve due to this
velocity of Mars on its axis, will this curve fit that one which
appears on Lowell's map, and of which we have really availed
ourselves of only three points? To answer this question we plot
upon a sphere, the curve of a satellite, in the manner I have
described, assigning to this sphere the velocity derived from the
span of 255 degrees. Having plotted the curve on the sphere it
only remains to transfer it to Lowell's map. This is easily
done.

192

This map (Pl. XXII) shows you the result of treating this, as
well as other curves, in the manner just described. You see that
whether the fragmentary curves are steep and receding far from
the equator; or whether they are flat and lying close along the
equator; whether they span less or more than 180 degrees; the
curves determined on the supposition that they are the work of
satellites revolving round Mars agree with the mapped curves;
following them with wonderful accuracy; possessing their
properties, and, indeed, in some cases, actually coinciding with
them.

I may add that the inadmissible span of 180 degrees and spans
very near this value, which are not well admissible, are so far
as I can find, absent. The curves are not great circles.

You will require of me that I should explain the centres of
radiation so conspicuous here and there on Lowell's map. The
meeting of more than two lines at the oases is a phenomenon
possibly of the same nature and also requiring explanation.

In the first place the curves to which I have but briefly
referred actually give rise in most cases to nodal, or crossing
points; sometimes on the equator, sometimes off the equator;
through which the path of the satellite returns again and again.
These nodal points will not, however, afford a general
explanation of the many-branched radiants.

It is probable that we should refer such an appearance

193

as is shown at the Sinus Titanum to the perturbations of the
satellite's path due to the surface features on Mars. Observe
that the principal radiants are situated upon the boundary of the
dark regions or at the oases. Higher surface levels may be
involved in both cases. Some marked difference in topography must
characterise both these features. The latter may possibly
originate in the destruction of satellites. Or again, they may
arise in crustal disturbance of a volcanic nature, primarily
induced or localised by the crossing of two canals. Whatever the
origin of these features it is only necessary to assume that they
represent elevated features of some magnitude to explain the
multiplication of crossing lines. We must here recall what
observers say of the multiplicity of the canals. According to
Lowell, "What their number maybe lies quite beyond the
possibility of count at present; for the better our own air, the
more of them are visible."

Such innumerable canals are just what the present theory
requires. An in-falling satellite will, in the course of the last
60 or 80 years of its career, circulate some 100,000 times over
Mars' surface. Now what will determine the more conspicuous
development of a particular canal? The mass of the satellite; the
state of the surface crust; the proximity of the satellite; and
the amount of repetition over the same ground. The after effects
may be taken as proportional to the primary disturbance.

194

It is probable that elevated surface features will influence two
of these conditions: the number of repetitions and the proximity
to the surface. A tract 100 miles in diameter and elevated 5,000
or 10,000 feet would seriously perturb the orbit of such a body as
Phobos. It is to be expected that not only would it be effective
in swaying the orbit of the satellite in the horizontal direction
but also would draw it down closer to the surface. It is even to
be considered if such a mass might not become nodal to the
satellite's orbit, so that this passed through or above this
point at various inclinations with its primary direction. If
acting to bring down the orbit then this will quicken the speed
and cause the satellite further on its path to attain a somewhat
higher elevation above the surface. The lines most conspicuous in
the telescope are, in short, those which have been favoured by a
combination of circumstances as reviewed above, among which
crustal features have, in some cases, played a part.

I must briefly refer to what is one of the most interesting
features of the Martian lines: the manner in which they appear to
come and go like visions.

Something going on in Mars determines the phenomenon. On a
particular night a certain line looks single. A few nights later
signs of doubling are perceived, and later still, when the seeing
is particularly good, not one but two lines are seen. Thus, as an
example, we may take the case of Phison and Euphrates. Faint
glimpses of the dual state were detected in the summer

195

and autumn, but not till November did they appear as distinctly
double. Observe that by this time the Antarctic snows had melted,
and there was in addition, sufficient time for the moisture so
liberated to become diffused in the planet's atmosphere.

This increase in the definition and conspicuousness of certain
details on Mars' surface is further brought into connection with
the liberation of the polar snows and the diffusion of this water
through the atmosphere, by the fact that the definition appeared
progressively better from the south pole upwards as the snow
disappeared. Lowell thinks this points to vegetation springing up
under the influence of moisture; he considers, however, as we
have seen, that the canals convey the moisture. He has to assume
the construction of triple canals to explain the doubling of the
lines.

If we once admit the canals to be elevated ranges—not necessarily
of great height—the difficulty of accounting for increased
definition with increase of moisture vanishes. We need not
necessarily even suppose vegetation concerned. With respect to
this last possibility we may remark that the colour observations,
upon which the idea of vegetation is based, are likely to be
uncertain owing to possible fatigue effects where a dark object
is seen against a reddish background.

However this may be we have to consider what the effects of
moisture increasing in the atmosphere of Mars will be with regard
to the visibility of elevated ranges,

196

We assume a serene and rare atmosphere: the nights intensely
cold, the days hot with the unveiled solar radiation. On the hill
tops the cold of night will be still more intense and so, also,
will the solar radiation by day. The result of this state of
things will be that the moisture will be precipitated mainly on
the mountains during the cold of night—in the form of frost—and
during the day this covering of frost will melt; and, just as we
see a heavy dew-fall darken the ground in summer, so the melting
ice will set off the elevated land against the arid plains below.
Our valleys are more moist than our mountains only because our
moisture is so abundant that it drains off the mountains into the
valleys. If moisture was scarce it would distil from the plains
to the colder elevations of the hills. On this view the
accentuation of a canal is the result of meteorological effects
such as would arise in the Martian climate; effects which must be
influenced by conditions of mountain elevation, atmospheric
currents, etc. We, thus, follow Lowell in ascribing the
accentuation of the canals to the circulation of water in Mars;
but we assume a simple and natural mode of conveyance and do not
postulate artificial structures of all but impossible magnitude.
That vegetation may take part in the darkening of the elevated
tracts is not improbable. Indeed we would expect that in the
Martian climate these tracts would be the only fertile parts of
the surface.

Clouds also there certainly are. More recent observations

197

appear to have set this beyond doubt. Their presence obviously
brings in other possible explanations of the coming and going of
elevated surface features.

Finally, we may ask what about the reliability of the maps? About
this it is to be said that the most recent map—that by Lowell—has
been confirmed by numerous drawings by different observers, and
that it is,itself the result of over 900 drawings. It has become
a standard chart of Mars, and while it would be rash to contend
for absence of errors it appears certain that the trend of the
principal canals may be relied on, as, also, the general features
of the planet's surface.

The question of the possibility of illusion has frequently been
raised. What I have said above to a great extent answers such
objections. The close agreement between the drawings of different
observers ought really to set the matter at rest. Recently,
however, photography has left no further room for scepticism.
First photographed in 1905, the planet has since been
photographed many thousands of times from various observatories.
A majority of the canals have been so mapped. The doubling of the
canals is stated to have been also so recorded.[1]

The hypothesis which I have ventured to put before you involves
no organic intervention to account for the

[1] E. C. Slipher's paper in _Popular Astronomy_ for March, 1914,
gives a good account of the recent work.

198

details on Mars' surface. They are physical surface features.
Mars presents his history written upon his face in the scars of
former encounters—like the shield of Sir Launcelot. Some of the
most interesting inferences of mathematical and physical
astronomy find a confirmation in his history. The slowing down in
the rate of axial rotation of the primary; the final inevitable
destruction of the satellite; the existence in the past of a far
larger number of asteroids than we at present are acquainted
with; all these great facts are involved in the theory now
advanced. If justifiably, then is Mars' face a veritable
Principia.

To fully answer the question which heads these lectures, we
should go out into the populous solitudes (if the term be
permitted) which lie beyond our system. It is well that there is
now no time left to do so; for, in fact, there we can only dream
dreams wherein the limits of the possible and the impossible
become lost.

The marvel of the infinite number of stars is not so marvellous
as the rationality that fain would comprehend them. In seeking
other minds than ours we seek for what is almost infinitely
complex and coordinated in a material universe relatively simple
and heterogeneous. In our mental attitude towards the great
question, this fact must be regarded as fundamental.

I can only fitly close a discourse which has throughout weighed
the question of the living thought against the unthinking laws of
matter, by a paraphrase of the words

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of a great poet when he, in higher and, perhaps, more philosophic
language, also sought to place the one in comparison with the
other.[1]

Richter thought that he was—with his human heart
unstrengthened—taken by an angel among the universe of stars.
Then, as they journeyed, our solar system was sunken like a faint
star in the abyss, and they travelled yet further, on the wings
of thought, through mightier systems: through all the countless
numbers of our galaxy. But at length these also were left behind,
and faded like a mist into the past. But this was not all. The
dawn of other galaxies appeared in the void. Stars more countless
still with insufferable light emerged. And these also were
passed. And so they went through galaxies without number till at
length they stood in the great Cathedral of the Universe. Endless
were the starry aisles; endless the starry columns; infinite the
arches and the architraves of stars. And the poet saw the mighty
galaxies as steps descending to infinity, and as steps going up
to infinity.

Then his human heart fainted and he longed for some narrow cell;
longed to lie down in the grave that he might hide from infinity.
And he said to the angel:

"Angel, I can go with thee no farther. Is there, then, no end to
the universe of stars?"

[1] De Quincy in his _System of the Heavens_ gives a fine
paraphrase of "Richter's Dream."

200

Then the angel flung up his glorious hands to the heaven of
heavens, saying "End is there none to the universe of God? Lo!
also there is no beginning."

201

THE LATENT IMAGE [1]

My inclination has led me, in spite of a lively dread of
incurring a charge of presumption, to address you principally on
that profound and most subtle question, the nature and mode of
formation of the photographic image. I am impelled to do so, not
only because the subject is full of fascination and hopefulness,
but because the wide topics of photographic methods or
photographic applications would be quite unfittingly handled by
the president you have chosen.

I would first direct your attention to Sir James Dewar's
remarkable result that the photographic plate retains
considerable power of forming the latent image at temperatures
approaching the absolute zero—a result which, as I submit,
compels us to regard the fundamental effects progressing in the
film under the stimulus of light undulations as other than those
of a purely chemical nature. But few, if any, instances of
chemical combination or decomposition are known at so low a
temperature. Purely chemical actions cease, indeed, at far higher
temperatures, fluorine being among the few bodies which still
show

[1] Presidential address to the Photographic Convention of the
United Kingdom, July, 1905. _Nature_, Vol. 72, p. 308.

202

chemical activity at the comparatively elevated temperature of
-180° C. In short, this result of Sir James Dewar's suggests that
we must seek for the foundations of photographic action in some
physical or intra-atomic effect which, as in the case of
radioactivity or fluorescence, is not restricted to intervals of
temperature over which active molecular vis viva prevails. It
compels us to regard with doubt the role of oxidation or other
chemical action as essential, but rather points to the view that
such effects must be secondary or subsidiary. We feel, in a word,
that we must turn for guidance to some purely photo-physical
effect.

Here, in the first place, we naturally recall the views of Bose.
This physicist would refer the formation of the image to a strain
of the bromide of silver molecule under the electric force in the
light wave, converting it into what might be regarded as an
allotropic modification of the normal bromide which subsequently
responds specially to the attack of the developer. The function
of the sensitiser, according to this view, is to retard the
recovery from strain. Bose obtained many suggestive parallels
between the strain phenomena he was able to observe in silver and
other substances under electromagnetic radiation and the
behaviour of the photographic plate when subjected to
long-continued exposure to light.

This theory, whatever it may have to recommend it, can hardly be
regarded as offering a fundamental explanation. In the first
place, we are left in the dark as to what

203

the strain may be. It may mean many and various things. We know
nothing as to the inner mechanism of its effects upon subsequent
chemical actions—or at least we cannot correlate it with what is
known of the physics of chemical activity. Finally, as will be
seen later, it is hardly adequate to account for the varying
degrees of stability which may apparently characterise the latent
image. Still, there is much in Bose's work deserving of careful
consideration. He has by no means exhausted the line of
investigation he has originated.

Another theory has doubtless been in the minds of many. I have
said we must seek guidance in some photo-physical phenomenon.
There is one such which preeminently connects light and chemical
phenomena through the intermediary of the effects of the former
upon a component part of the atom. I refer to the phenomena of
photo-electricity.

It was ascertained by Hertz and his immediate successors that
light has a remarkable power of discharging negative
electrification from the surface of bodies—especially from
certain substances. For long no explanation of the cause of this
appeared. But the electron—the ubiquitous electron—is now known
with considerable certainty to be responsible. The effect of the
electric force in the light wave is to direct or assist the
electrons contained in the substance to escape from the surface
of the body. Each electron carries away a very small charge of
negative electrification. If, then, a body is

204

originally charged negatively, it will be gradually discharged by
this convective process. If it is not charged to start with, the
electrons will still be liberated at the surface of the body, and
this will acquire a positive charge. If the body is positively
charged at first, we cannot discharge it by illumination.

It would be superfluous for me to speak here of the nature of
electrons or of the various modes in which their presence may be
detected. Suffice it to say, in further connection with the Hertz
effect, that when projected among gaseous molecules the electron
soon attaches itself to one of these. In other words, it ionises
a molecule of the gas or confers its electric charge upon it. The
gaseous molecule may even be itself disrupted by impact of the
electron, if this is moving fast enough, and left bereft of an
electron.

We must note that such ionisation may be regarded as conferring
potential chemical properties upon the molecules of the gas and
upon the substance whence the electrons are derived. Similar
ionisation under electric forces enters, as we now believe, into
all the chemical effects progressing in the galvanic cell, and,
indeed, generally in ionised solutes.

An experiment will best illustrate the principles I wish to
remind you of. A clean aluminium plate, carefully insulated by a
sulphur support, is faced by a sheet of copper-wire-gauze placed
a couple of centimetres away from it. The gauze is maintained at
a high positive

205

potential by this dry pile. A sensitive gold-leaf electroscope is
attached to the aluminium plate, and its image thrown upon the
screen. I now turn the light from this arc lamp upon the wire
gauze, through which it in part passes and shines upon the
aluminium plate. The electroscope at once charges up rapidly.
There is a liberation of negative electrons at the surface of the
aluminium; these, under the attraction of the positive body, are
rapidly removed as ions, and the electroscope charges up
positively.

Again, if I simply electrify negatively this aluminium plate so
that the leaves of the attached electroscope diverge widely, and
now expose it to the rays from the arc lamp, the charge, as you
see, is very rapidly dissipated. With positive electrification of
the aluminium there is no effect attendant on the illumination.

Thus from the work of Hertz and his successors we know that
light, and more particularly what we call actinic light, is an
effective means of setting free electrons from certain
substances. In short, our photographic agent, light, has the
power of expelling from certain substances the electron which is
so potent a factor in most, if not in all, chemical effects. I
have not time here to refer to the work of Elster and Geitel
whereby they have shown that this action is to be traced to the
electric force in the light wave, but must turn to the probable
bearing of this phenomenon on the familiar facts of photography.
I assume that the experiment I have shown you is the most

206

fundamental photographic experiment which it is now in our power
to make.

We must first ask from what substances can light liberate
electrons. There are many—metals as well as non-metals and
liquids. It is a very general phenomenon and must operate widely
throughout nature. But what chiefly concerns the present
consideration is the fact that the haloid salts of silver are
vigorously photo-electric, and, it is suggestive, possess,
according to Schmidt, an activity in the descending order
bromide, chloride, iodide. This is, in other words, their order
of activity as ionisers (under the proper conditions) when
exposed to ultra-violet light. Photographers will recognise that
this is also the order of their photographic sensitiveness.

Another class of bodies also concerns our subject: the special
sensitisers used by the photographer to modify the spectral
distribution of sensibility of the haloid salts, _e.g._ eosine,
fuchsine, cyanine. These again are electron-producers under light
stimulus. Now it has been shown by Stoletow, Hallwachs, and
Elster and Geitel that there is an intimate connection between
photo-electric activity and the absorption of light by the
substance, and, indeed, that the particular wave-lengths absorbed
by the substance are those which are effective in liberating the
electrons. Thus we have strong reason for believing that the
vigorous photo-electric activity displayed by the special
sensitisers must be dependent upon their colour absorption. You
will recognise that this is just

207

the connection between their photographic effects and their
behaviour towards light.

There is yet another suggestive parallel. I referred to the
observation of Sir James Dewar as to the continued sensitiveness
of the photographic film at the lowest attained extreme of
temperature, and drew the inference that the fundamental
photographic action must be of intra-atomic nature, and not
dependent upon the vis viva of the molecule or atom. In then
seeking the origin of photographic action in photo-electric
phenomena we naturally ask, Are these latter phenomena also
traceable at low temperatures? If they are, we are entitled to
look upon this fact as a qualifying characteristic or as another
link in the chain of evidence connecting photographic with
photo-electric activity.

I have quite recently, with the aid of liquid air supplied to me
from the laboratory of the Royal Dublin Society, tested the
photo-sensibility of aluminium and also of silver bromide down to
temperatures approaching that of the liquid air. The mode of
observation is essentially that of Schmidt—what he terms his
static method. The substance undergoing observation is, however,
contained at the bottom of a thin copper tube, 5 cm. in diameter,
which is immersed to a depth of about 10 cm in liquid air. The
tube is closed above by a paraffin stopper which carries a thin
quartz window as well as the sulphur tubes through which the
connections pass. The air within is very carefully dried by
phosphorus

208

pentoxide before the experiment. The arc light is used as source
of illumination. It is found that a vigorous photo-electric
effect continues in the case of the clean aluminium. In the case
of the silver bromide a distinct photo-electric effect is still
observed. I have not had leisure to make, as yet, any trustworthy
estimate of the percentage effect at this temperature in the case
of either substance. Nor have I determined the temperature
accurately. The latter may be taken as roughly about -150° C,

Sir James Dewar's actual measilrements afforded twenty per cent.
of the normal photographic effect at -180° C. and ten per cent.
at the temperature of -252.5° C.

With this much to go upon, and the important additional fact that
the electronic discharge—as from the X-ray tube or from
radium—generates the latent image, I think we are fully entitled
to suggest, as a legitimate lead to experiment, the hypothesis
that the beginnings of photographic action involve an electronic
discharge from the light-sensitive molecule; in other words that
the latent image is built up of ionised atoms or molecules the
result of the photo-electric effect on the illuminated silver
haloid, and it is upon these ionised atoms that the chemical
effects of the developer are subsequently directed. It may be
that the liberated electrons ionise molecules not directly
affected, or it may be that in their liberation they disrupt
complex molecules built up in the ripening of the

209

emulsion. With the amount we have to go upon we cannot venture to
particularise. It will be said that such an action must be in
part of the nature of a chemical effect. This must be admitted,
and, in so far as the rearrangement of molecular fabrics is
involved, the result will doubtless be controlled by temperature
conditions. The facts observed by Sir James Dewar support this.
But there is involved a fundamental process—the liberation of the
electron by the electric force in the light wave, which is a
physical effect, and which, upon the hypothesis of its reality as
a factor in forming the latent image, appears to explain
completely the outstanding photographic sensitiveness of the film
at temperatures far below those at which chemical actions in
general cease.

Again, we may assume that the electron—producing power of the
special sensitiser or dye for the particular ray it absorbs is
responsible, or responsible in part, for the special
sensitiveness it confers upon the film. Sir Wm. Abney has shown
that these sensitisers are active even if laid on as a varnish on
the sensitive surface and removed before development. It must be
remembered, however, that at temperatures of about -50° these
sensitisers lose much of their influence on the film; as I have
pointed out in a paper read before the Photographic Convention of
1894.

It. appears to me that on these views the curious phenomenon of
recurrent reversals does not present a problem hopeless of
explanation. The process of photo-

210

ionisation constituting the latent image, where the ion is
probably not immediately neutralised by chemical combination,
presents features akin to the charging of a capacity—say a Leyden
jar. There may be a rising potential between the groups of ions
until ultimately a point is attained when there is a spontaneous
neutralisation. I may observe that the phenomena of reversal
appear to indicate that the change in the silver bromide
molecule, whatever be its nature, is one of gradually increasing
intensity, and finally attains a maximum when a return to the
original condition occurs. The maximum is the point of most
intense developable image. It is probable that the sensitiser—in
this case the gelatin in which the bromide of silver is
immersed—plays a part in the conditions of stability which are
involved.

Of great interest in all our considerations and theories is the
recent work of Wood on photographic reversal. The result of this
work is—as I take it—to show that the stability of the latent
image may be very various according to the mode of its formation.
Thus it appears that the sort of latent effect which is produced
by pressure or friction is the least stable of any. This may be
reversed or wiped out by the application of any other known form
of photographic stimulus. Thus an exposure to X-rays will
obliterate it, or a very brief exposure to light. The latent
image arising from X-rays is next in order of increasing
stability. Light action will remove this. Third in order is a
very brief light-shock or sudden flash. This

211

cannot be reversed by any of the foregoing modes of stimulation,
but a long-continued undulatory stimulus, as from lamp-light,
will reverse it. Last and most stable of all is the gradually
built-up configuration due to long-continued light exposure. This
can only be reversed by overdoing it according to the known facts
of recurrent reversal. Wood takes occasion to remark that these
phenomena are in bad agreement with the strain theory of Bose. We
have, in fact, but the one resource—the allotropic modification
of the haloid—whereby to explain all these orders of stability.
It appears to me that the elasticity of the electronic theory is
greater. The state of the ionised system may be very various
according as it arises from continued rhythmic effects or from
unorganised shocks. The ionisation due to X-rays or to friction
will probably be quite unorganised, that due to light more or
less stable according to the gradual and gentle nature of the
forces at work. I think we are entitled to conclude that on the
whole there is nothing in Wood's beautiful experiments opposed to
the photo-electric origin of photographic effects, but that they
rather fall in with what might be anticipated according to that
theory.

When we look for further support to the views I have laid before
you we are confronted with many difficulties. I have not as yet
detected any electronic discharge from the film under light
stimulus. This may be due to my defective experiments, or to a
fact noted by Elster and Geitel concerning the photo-electric
properties of gelatin.

212

They obtained a vigorous effect from Balmain's luminous paint,
but when this was mixed in gelatin there was no external effect.
Schmidt's results as to the continuance of photo-electric
activity when bodies in general are dissolved in each other lead
us to believe that an actual conservative property of the medium
and not an effect of this on the luminous paint is here involved.
This conservative effect of the gelatin may be concerned with its
efficacy as a sensitiser.

In the views I have laid before you I have endeavoured to show
that the recent addition to our knowledge of the electron as an
entity taking part in many physical and chemical effects should
be kept in sight in seeking an explanation of the mode of origin
of the latent image.[1]

[1] For a more detailed account of the subject, and some
ingenious extensions of the views expressed above, see
_Photo-Electricity_, by H. Stanley Allen: Longmans, Green & Ca.,
1913.

213

PLEOCHROIC HALOES [1]

IT is now well established that a helium atom is expelled from
certain of the radioactive elements at the moment of
transformation. The helium atom or alpha ray leaves the
transforming atom with a velocity which varies in the different
radioactive elements, but which is always very great, attaining
as much as 2 x 109 cms. per second; a velocity which, if
unchecked, would carry the atom round the earth in less than two
seconds. The alpha ray carries a positive charge of double the
ionic amount.

When an alpha ray is discharged from the transforming element
into a gaseous medium its velocity is rapidly checked and its
energy absorbed. A certain amount of energy is thus transferred
from the transforming atom to the gas. We recognise this energy
in the gas by the altered properties of the latter; chiefly by
the fact that it becomes a conductor of electricity. The
mechanism by which this change is effected is in part known. The
atoms of the gas, which appear to be freely penetrated by the
alpha ray, are so far dismembered as to yield charged electrons
or ions; the atoms remaining charged with an equal and opposite
charge. Such a medium of

[1] Being the Huxley Lecture, delivered at the University of
Birmingham on October 30th, 1912. Bedrock, Jan., 1913.

214

free electric charges becomes a conductor of electricity by
convection when an electromotive force is applied. The gas also
acquires other properties in virtue of its ionisation. Under
certain conditions it may acquire chemical activity and new
combinations may be formed or existing ones broken up. When its
initial velocity is expended the helium atom gives up its
properties as an alpha ray and thenceforth remains possessed of
the ordinary varying velocity of thermal agitation. Bragg and
Kleeman and others have investigated the career of the alpha ray
when its path or range lies in a gas at ordinary or obtainable
conditions of pressure and temperature. We will review some of
the facts ascertained.

The range or distance traversed in a gas at ordinary pressures is
a few centimetres. The following table, compiled by Geiger, gives
the range in air at the temperature of 15° C.:

               cms.                   cms.                   cms.
Uranium 1 -   2.50    Thorium -     2.72     Radioactinium  4.60
Uranium 2 -   2.90    Radiothorium  3.87     Actinium X -   4.40
Ionium -      3.00    Thorium X -   4.30     Act Emanation  5.70
Radium -      3.30    Th Emanation  5.00     Actinium A -   6.50
Ra Emanation  4.16    Thorium A -   5.70     Actinium C -   5.40
Radium A -    4.75    Thorium C1 -  4.80
Radium C -    6.94    Thorium C2 -  8.60
Radium F -    3.77

It will be seen that the ray of greatest range is that proceeding
from thorium C2, which reaches a distance of 8.6 cms. In the
uranium family the fastest ray is

215

that of radium C. It attains 6.94 cms. There is thus an
appreciable difference between the ultimate distances traversed
by the most energetic rays of the two families. The shortest
ranges are those of uranium 1 and 2.

The ionisation effected by these rays is by no means uniform
along the path of the ray. By examining the conductivity of the
gas at different points along the path of the ray, the ionisation
at these points may be determined. At the limits of the range the
ionisation

{Fig. 13}

ceases. In this manner the range is, in fact, determined. The
dotted curve (Fig. 13) depicts the recent investigation of the
ionisation effected by a sheaf of parallel rays of radium C in
air, as determined by Geiger. The range is laid out horizontally
in centimetres. The numbers of ions are laid out vertically. The
remarkable nature of the results will be at once apparent. We
should have expected that the ray at the beginning of its path,
when its velocity and kinetic energy were greatest, would have
been more effective than towards the end of its range

216

when its energy had almost run out. But the curve shows that it
is just the other way. The lagging ray, about to resign its
ionising properties, becomes a much more efficient ioniser than
it was at first. The maximum efficiency is, however, in the case
of a bundle of parallel rays, not quite at the end of the range,
but about half a centimetre from it. The increase to the maximum
is rapid, the fall from the maximum to nothing is much more
rapid.

It can be shown that the ionisation effected anywhere along the
path of the ray is inversely proportional to the velocity of the
ray at that point. But this evidently does not apply to the last
5 or 10 mms. of the range where the rate of ionisation and of the
speed of the ray change most rapidly. To what are the changing
properties of the rays near the end of their path to be ascribed?
It is only recently that this matter has been elucidated.

When the alpha ray has sufficiently slowed down, its power of
passing right through atoms, without appreciably experiencing any
effects from them, diminishes. The opposing atoms begin to exert
an influence on the path of the ray, deflecting it a little. The
heavier atoms will deflect it most. This effect has been very
successfully investigated by Geiger. It is known as "scattering."
The angle of scattering increases rapidly with the decrease of
velocity. Now the effect of the scattering will be to cause some
of the rays to complete their ranges

217

or, more accurately, to leave their direct line of advance a
little sooner than others. In the beautiful experiments of C. T.
R. Wilson we are enabled to obtain ocular demonstration of the
scattering. The photograph (Fig. 14.), which I owe to the
kindness of Mr. Wilson, shows the deflection of the ray towards
the end of its path. In

{Fig. 14}

this case the path of the ray has been rendered visible by the
condensation of water particles under the influence of the
ionisation; the atmosphere in which the ray travels being in a
state of supersaturation with water vapour at the instant of the
passage of the ray. It is evident that if we were observing the
ionisation along a sheaf of parallel rays, all starting with
equal velocity,

218

the effect of the bending of some of the rays near the end of
their range must be to cause a decrease in the aggregate
ionisation near the very end of the ultimate range. For, in fact,
some of the rays complete their work of ionising at points in the
gas before the end is reached. This is the cause, or at least an
important contributory cause, of the decline in the ionisation
near the end of the range, when the effects of a bundle of rays
are being observed. The explanation does not suggest that the
ionising power of any one ray is actually diminished before it
finally ceases to be an alpha ray.

The full line in Fig. 13 gives the ionisation curve which it may
be expected would be struck out by a single alpha ray. In it the
ionisation goes on increasing till it abruptly ceases altogether,
with the entire loss of the initial kinetic energy of the
particle.

A highly remarkable fact was found out by Bragg. The effect of
the atom traversed by the ray in checking the velocity of the ray
is independent of the physical and chemical condition of the
atom. He measured the "stopping power" of a medium by the
distance the ray can penetrate into it compared with the distance
to which it can penetrate in air. The less the ratio the greater
is the stopping power. The stopping power of a substance is
proportional to the square root of its atomic weight. The
stopping power of an atom is not altered if it is in chemical
union with another atom. The atomic weight is the one quality of
importance. The physical

219

state, whether the element is in the solid, liquid or gaseous
state, is unimportant. And when we deal with molecules the
stopping power is simply proportional to the sum of the square
roots of the atomic weights of the atoms entering into the
molecule. This is the "additive law," and it obviously enables us
to calculate what the range in any substance of known chemical
composition and density will be, compared with the range in air.

This is of special importance in connection with phenomena we
have presently to consider. It means that, knowing the chemical
composition and density of any medium whatsoever, solid, liquid
or gaseous, we can calculate accurately the distance to which any
particular alpha ray will penetrate. Nor have the temperature and
pressure to which the medium is subjected any influence save in
so far as they may affect the proximity of one atom to another.
The retardation of the alpha ray in the atom is not affected.

This valuable additive law, however, cannot be applied in
strictness to the amount of ionisation attending the ray. The
form of the molecule, or more generally its volume, may have an
influence upon this. Bragg draws the conclusion, from this fact
as well as from the notable increase of ionisation with loss of
speed, that the ionisation is dependent upon the time the ray
spends in the molecule. The energy of the ray is, indeed, found
to be less efficient in producing ionisation in the smaller
atomm.

220

Before leaving our review of the general laws governing the
passage of alpha rays through matter, another point of interest
must be referred to. We have hitherto spoken in general terms of
the fact that ionisation attends the passage of the ray. We have
said nothing as to the nature of the ionisation so produced. But
in point of fact the ionisation due to an alpha ray is sui
generis. A glance at one of Wilson's photographs (Fig. 14.)
illustrates this. The white streak of water particles marks the
path of the ray. The ions produced are evidently closely crowded
along the track of the ray. They have been called into existence
in a very minute instant of time. Now we know that ions of
opposite sign if left to themselves recombine. The rate of
recombination depends upon the product of the number of each sign
present in unit volume. Here the numbers are very great and the
volume very small. The ionic density is therefore high, and
recombination very rapidly removes the ions after they are
formed. We see here a peculiarity of the ionisation effected by
alpha rays. It is linear in distribution and very local. Much of
the ionisation in gases is again undone by recombination before
diffusion leads to the separation of the ions. This "initial
recombination" is greatest towards the end of the path of the ray
where the ionisation is a maximum. Here it may be so effective
that the form of the curve is completely lost unless a very large
electromotive force is used to separate the ions when the
ionisation is being investigated.

221

We have now reviewed recent work at sufficient length to
understand something of the nature of the most important advance
ever made in our knowledge of the atom. Let us glance briefly at
what we have learned. The radioactive atom in sinking to a lower
atomic weight casts out with enormous velocity an atom of helium.
It thus loses a definite portion of its mass and of its energy.
Helium which is chemically one of the most inert of the elements,
is, when possessed of such great kinetic energy, able to
penetrate and ionise the atoms which it meets in its path. It
spends its energy in the act of ionising them, coming to rest,
when it moves in air, in a few centimetres. Its initial velocity
depends upon the particular radioactive element which has given
rise to it. The length of its path is therefore different
according to the radioactive element from which it proceeds. The
retardation which it experiences in its path depends entirely
upon the atomic weight of the atoms which it traverses. As it
advances in its path its effectiveness in ionising the atom
rapidly increases and attains a very marked maximum. In a gas the
ions produced being much crowded together recombine rapidly; so
rapidly that the actual ionisation may be quite concealed unless
a sufficiently strong electric force is applied to separate them.
Such is a brief summary of the climax of radioactive
discovery:—the birth, life and death of the alpha ray. Its advent
into Science has altered fundamentally our conception of

222

matter. It is fraught with momentous bearings upon Geological
Science. How the work of the alpha ray is sometimes recorded
visibly in the rocks and what we may learn from that record, I
propose now to bring before you.

In certain minerals, notably the brown variety of mica known as
biotite, the microscope reveals minute circular marks occurring
here and there, quite irregularly. The most usual appearance is
that of a circular area darker in colour than the surrounding
mineral. The radii of these little disc-shaped marks when well
defined are found to be remarkably uniform, in some cases four
hundredths of a millimetre and in others three hundredths, about.
These are the measurements in biotite. In other minerals the
measurements are not quite the same as in biotite. Such minute
objects are quite invisible to the naked eye. In some rocks they
are very abundant, indeed they may be crowded together in such
numbers as to darken the colour of the mineral containing them.
They have long been a mystery to petrologists.

Close examination shows that there is always a small speck of a
foreign body at the centre of the circle, and it is often
possible to identify the nature of this central substance, small
though it be. Most generally it is found to be the mineral
zircon. Now this mineral was shown by Strutt to contain radium in
quantities much exceeding those found in ordinary rock
substances.

223

Some other mineral may occasionally form the nucleus, but we
never find any which is not known to be specially likely to
contain a radioactive substance. Another circumstance we notice.
The smaller this central nucleus the more perfect in form is the
darkened circular area surrounding it. When the circle is very
perfect and the central mineral clearly defined at its centre we
find by measurement that the radius of the darkened area is
generally 0.033 mm. It may sometimes be 0.040 mm. These are
always the measurements in biotite. In other minerals the radii
are a little different.

We see in the photograph (Pl. XXIII, lower figure), much
magnified, a halo contained in biotite. We are looking at a
region in a rock-section, the rock being ground down to such a
thickness that light freely passes through it. The biotite is in
the centre of the field. Quartz and felspar surround it. The rock
is a granite. The biotite is not all one crystal. Two crystals,
mutually inclined, are cut across. The halo extends across both
crystals, but owing to the fact that polarised light is used in
taking the photograph it appears darker in one crystal than in
the other. We see the zircon which composes the nucleus. The fine
striated appearance of the biotite is due to the cleavage of that
mineral, which is cut across in the section.

The question arises whether the darkened area surrounding the
zircon may not be due to the influence of the radioactive
substances contained in the zircon. The

224

extraordinary uniformity of the radial measurements of perfectly
formed haloes (to use the name by which they have long been
known) suggests that they may be the result of alpha radiation.
For in that case, as we have seen, we can at once account for the
definite radius as simply representing the range of the ray in
biotite. The furthest-reaching ray will define the radius of the
halo. In the case of the uranium family this will be radium C,
and in the case of thorium it will be thorium C. Now here we
possess a means of at once confirming or rejecting the view that
the halo is a radioactive phenomenon and occasioned by alpha
radiation; for we can calculate what the range of these rays will
be in biotite, availing ourselves of Bragg's additive law,
already referred to. When we make this calculation we find that
radium C just penetrates 0.033 mm. and thorium C 0.040 mm. The
proof is complete that we are dealing with the effects of alpha
rays. Observe now that not only is the coincidence of measurement
and calculation a proof of the view that alpha radiation has
occasioned the halo, but it is a very complete verification of
the important fact stated by Bragg, that the stopping power
depends solely on the atomic weight of the atoms traversed by the
ray.

We have seen that our examination of the rocks reveals only the
two sorts of halo: the radium halo and the thorium halo. This is
not without teaching. For why not find an actinium halo? Now
Rutherford long ago suggested that this element and its
derivatives were

225

probably an offspring of the uranium family; a side branch, as it
were, in the formation of which relatively few transforming atoms
took part. On Rutherford's theory then, actinium should always
accompany uranium and radium, but in very subordinate amount. The
absence of actinium haloes clearly supports this view. For if
actinium was an independent element we would be sure to find
actinium haloes. The difference in radius should be noticeable.
If, on the other hand, actinium

was always associated with uranium and radium, then its effects
would be submerged in those of the much more potent effects of
the uranium series of elements.

It will have occurred to you already that if the radioactive
origin of the halo is assured the shape of a halo is not really
circular, but spherical. This is so. There is no such thing as a
disc-shaped halo. The halo is a spherical volume containing the
radioactive nucleus at its centre. The true radius of the halo
may, therefore, only be measured on sections passing through the
nucleus.

226

In order to understand the mode of formation of a halo we may
profitably study on a diagram the events which go on within the
halo-sphere. Such a diagram is seen in Fig. 15. It shows to
relatively correct scale the limiting range of all the alpha-ray
producing members of the uranium and thorium families. We know
that each member of a family will exist in equilibrium amount
within the nucleus possessing the parent element. Each alpha ray
leaving the nucleus will just attain its range and then cease to
affect the mica. Within the halosphere, there must be, therefore,
the accumulated effects of the influences of all the rays. Each
has its own sphere of influence, and the spheres are all
concentric.

The radii in biotite of the several spheres are given in the
following table

URANIUM FAMILY.
Radium C -       0.0330 mm.
Radium A -       0.0224 mm.
Ra Emanation -   0.0196 mm.
Radium F -       0.0177 mm.
Radium -         0.0156 mm.
Ionium -         0.0141 mm.
Uranium 1 -      0.0137 mm.
Uranium 2 -      0.0118 mm.

THORIUM FAMILY.
Thorium CE -     0.040 mm.
Thorium A -      0.026 mm.
Th Emanation -   0.023 mm.
Thorium Ci -     0.022 mm.
Thorium X -      0.020 mm.
Radiothorium -   0.119 mm.
Thorium -        0.013 mm.

In the photograph (Pl. XXIV, lower figure), we see a uranium and
a thorium halo in the same crystal of mica. The mica is contained
in a rock-section and is cut across the cleavage. The effects of
thorium Ca are clearly shown

227

as a lighter border surrounding the accumulated inner darkening
due to the other thorium rays. The uranium halo (to the right)
similarly shows the effects of radium C, but less distinctly.

Haloes which are uniformly dark all over as described above are,
in point of fact, "over-exposed"; to borrow a familiar
photographic term. Haloes are found which show much beautiful
internal detail. Too vigorous action obscures this detail just as
detail is lost in an over-exposed photograph. We may again have
"under-exposed" haloes in which the action of the several rays is
incomplete or in which the action of certain of the rays has left
little if any trace. Beginning at the most under-exposed haloes
we find circular dark marks having the radius 0.012 or 0.013 mm.
These haloes are due to uranium, although their inner darkening
is doubtless aided by the passage of rays which were too few to
extend the darkening beyond the vigorous effects of the two
uranium rays. Then we find haloes carried out to the radii 0.016,
0.018 and 0.019 mm. The last sometimes show very beautiful outer
rings having radial dimensions such as would be produced by
radium A and radium C. Finally we may have haloes in which
interior detail is lost so far out as the radius due to emanation
or radium A, while outside this floats the ring due to radium C.
Certain variations of these effects may occur, marking,
apparently, different stages of exposure. Plates XXIII and XXIV
(upper figure) illustrate some of these stages;

228

the latter photograph being greatly enlarged to show clearly the
halo-sphere of radium A.

In most of the cases mentioned above the structure evidently
shows the existence of concentric spherical shells of darkened
biotite. This is a very interesting fact. For it proves that in
the mineral the alpha ray gives rise to the same increased
ionisation towards the end of its range, as Bragg determined in
the case of gases. And we must conclude that the halo in every
case grows in this manner. A spherical shell of darkened biotite
is first produced and the inner colouration is only effected as
the more feeble ionisation along the track of the ray in course
of ages gives rise to sufficient alteration of the mineral. This
more feeble ionisation is, near the nucleus, enhanced in its
effects by the fact that there all the rays combine to increase
the ionisation and, moreover, the several tracks are there
crowded by the convergency to the centre. Hence the most
elementary haloes seldom show definite rings due to uranium,
etc., but appear as embryonic disc-like markings. The photographs
illustrate many of the phases of halo development.

Rutherford succeeded in making a halo artificially by compressing
into a capillary glass tube a quantity of the emanation of
radium. As the emanation decayed the various derived products
came into existence and all the several alpha rays penetrated the
glass, darkening the walls of the capillary out to the limit of
the range of radium C in glass. Plate XXV shows a magnified
section of the

229

tube. The dark central part is the capillary. The tubular halo
surrounds it. This experiment has, however, been anticipated by
some scores of millions of years, for here is the same effect in
a biotite crystal (Pl. XXV). Along what are apparently tubular
passages or cracks in the mica, a solution, rich in radioactive
substances, has moved; probably during the final consolidation of
the granite in which the mica occurs. A continuous and very
regular halo has developed along these conduits. A string of
halo-spheres may lie along such passages. We must infer that
solutions or gases able to establish the radioactive nuclei moved
along these conduits, and we are entitled to ask if all the
haloes in this biotite are not, in this sense, of secondary
origin. There is, I may add, much to support such a conclusion.