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.
The widespread distribution of radioactive substances is most
readily appreciated by examination of sections of rocks cut thin
enough for microscopic investigation. It is, indeed, difficult to
find, in the older rocks of granitic type, mica which does not
show haloes, or traces of haloes. Often we find that every one of
the inclusions in the mica--that is, every one of the earlier
formed substances--contain radioactive elements, as indicated by
the presence of darkened borders. As will be seen presently the
quantities involved are generally vanishingly small. For example
it was found by direct determination that in one gram of the
halo-rich mica of Co. Carlow there was rather less than twelve
billionths of a gram of radium, We are
230
entitled to infer that other rare elements are similarly widely
distributed but remain undetectable because of their more stable
properties.
It must not be thought that the under-exposed halo is a recent
creation. By no means. All are old, appallingly old; and in the
same rock all are, probably, of the same, or neatly the same,
age. The under-exposure is simply due to a lesser quantity of the
radioactive elements in the nucleus. They are under-exposed, in
short, not because of lesser duration of exposure, but because of
insufficient action; as when in taking a photograph the stop is
not open enough for the time of the exposure.
The halo has, so far, told us that the additive law is obeyed in
solid media, and that the increased ionisation attending the
slowing down of the ray obtaining in gases, also obtains in
solids; for, otherwise, the halo would not commence its
development as a spherical shell or envelope. But here we learn
that there is probably a certain difference in the course of
events attending the immediate passage of the ray in the gas and
in the solid. In the former, initial recombination may obscure
the intense ionisation near the end of the range. We can only
detect the true end-effects by artificially separating the ions
by a strong electric force. If this recombination happened in the
mineral we should not have the concentric spheres so well defined
as we see them to be. What, then, hinders the initial
recombination in the solid? The answer probably is that the newly
formed
231
ion is instantly used up in a fresh chemical combination. Nor is
it free to change its place as in the gas. There is simply a new
equilibrium brought about by its sudden production. In this
manner the conditions in the complex molecule of biotite,
tourmaline, etc., may be quite as effective in preventing initial
recombination as the most effective electric force we could
apply. The final result is that we find the Bragg curve
reproduced most accurately in the delicate shading of the rings
making up the perfectly exposed halo.
That the shading of the rings reproduces the form of the Bragg
curve, projected, as it were, upon the line of advance of the ray
and reproduced in depth of shading, shows that in yet another
particular the alpha ray behaves much the same in the solid as in
the gas. A careful examination of the outer edge of the circles
always reveals a steep but not abrupt cessation of the action of
the ray. Now Geiger has investigated and proved the existence of
scattering of the alpha ray by solids. We may, therefore, suppose
with much probability that there is the same scattering within
the mineral near the end of the range. The heavy iron atom of the
biotite is, doubtless, chiefly responsible for this in biotite
haloes. I may observe that this shading of the outer bounding
surface of the sphere of action is found however minute the
central nucleus. In the case of a nucleus of considerable size
another effect comes in which tends to produce an enhanced
shading. This will
232
result if rays proceed from different depths in the nucleus. If
the nucleus were of the same density and atomic weight as the
surrounding mica, there would be little effect. But its density
and molecular weight are generally greater, hence the retardation
is greater, and rays proceeding from deep in the nucleus
experience more retardation than those which proceed from points
near to the surface. The distances reached by the rays in the
mica will vary accordingly, and so there will be a gradual
cessation of the effects of the rays.
The result of our study of the halo may be summed up in the
statement that in nearly every particular we have the phenomena,
which have been measured and observed in the gas, reproduced on a
minute scale in the halo. Initial recombination seems, however,
to be absent or diminished in effectiveness; probably because of
the new stability instantly assumed by the ionised atoms.
One of the most interesting points about the halo remains to be
referred to. The halo is always uniformly darkened all round its
circumference and is perfectly spherical. Sections, whether taken
in the plane of cleavage of the mica or across it, show the same
exactly circular form, and the same radius. Of course, if there
was any appreciable increase of range along or across the
cleavage the form of the halo on the section across the cleavage
should be elliptical. The fact that there is no measurable
ellipticity is, I think, one which on first consideration would
not be expected.
233
For what are the conditions attending the passage of the ray in a
medium such as mica? According to crystallographic conceptions we
have here an orderly arrangement of molecules, the units
composing the crystal being alike in mass, geometrically spaced,
and polarised as regards the attractions they exert one upon
another. Mica, more especially, has the cleavage phenomenon
developed to a degree which transcends its development in any
other known substance. We can cleave it and again cleave it till
its flakes float in the air, and we may yet go on cleaving it by
special means till the flakes no longer reflect visible light.
And not less remarkable is the uniplanar nature of its cleavage.
There is little cleavage in any plane but the one, although it is
easy to show that the molecules in the plane of the flake are in
orderly arrangement and are more easily parted in some directions
than in others. In such a medium beyond all others we must look
with surprise upon the perfect sphere struck out by the alpha
rays, because it seems certain that the cleavage is due to lesser
attraction, and, probably, further spacing of the molecules, in a
direction perpendicular to the cleavage.
It may turn out that the spacing of the molecules will influence
but little the average number per unit distance encountered by
rays moving in divergent paths. If this is so, we seem left to
conclude that, in spite of its unequal and polarised attractions,
there is equal retardation and equal ionisation in the molecule
in whatever
234
direction it is approached. Or, again, if the encounters indeed
differ in number, then some compensating effect must exist
whereby a direction of lesser linear density involves greater
stopping power in the molecule encountered, and vice versa.
The nature of the change produced by the alpha rays is unknown.
But the formation of the halo is not, at least in its earlier
stages, attended by destruction of the crystallographic and
optical properties of the medium. The optical properties are
unaltered in nature but are increased in intensity. This applies
till the halo has become so darkened that light is no longer
transmitted under the conditions of thickness obtaining in rock
sections. It is well known that there is in biotite a maximum
absorption of a plane-polarised light ray, when the plane of
vibration coincides with the plane of cleavage. A section across
the cleavage then shows a maximum amount of absorption. A halo
seen on this section simply produces this effect in a more
intense degree. This is well shown in Plate XXIII (lower figure),
on a portion of the halo-sphere. The descriptive name "Pleochroic
Halo" has originated from this fact. We must conclude that the
effect of the ionisation due to the alpha ray has not been to
alter fundamentally the conditions which give rise to the optical
properties of the medium. The increased absorption is probably
associated with some change in the chemical state of the iron
present. Haloes are, I believe, not found in minerals from which
this
235
element is absent. One thing is quite certain. The colouration is
not due to an accumulation of helium atoms, _i.e._ of spent alpha
rays. The evidence for this is conclusive. If helium was
responsible we should have haloes produced in all sorts of
colourless minerals. Now we sometimes see zircons in felspars and
in quartz, etc., but in no such case is a halo produced. And
halo-spheres formed within and sufficiently close to the edge of
a crystal of mica are abruptly truncated by neighbouring areas of
fclspar or quartz, although we know that the rays must pass
freely across the boundary. Again it is easy to show that even in
the oldest haloes the quantity of helium involved is so small
that one might say the halo-sphere was a tolerably good vacuum as
regards helium. There is, finally, no reason to suppose that the
imprisoned helium would exhibit such a colouration, or, indeed,
any at all.
I have already referred to the great age of the halo. Haloes are
not found in the younger igneous rocks. It is probable that a
halo less than a million years old has never been seen. This,
primâ facie, indicates an extremely slow rate of formation. And
our calculations quite support the conclusions that the growth of
a halo, if this has been uniform, proceeds at a rate of almost
unimaginable slowness.
Let us calculate the number of alpha rays which may have gone to
form a halo in the Devonian granite of Leinster.
236
It is common to find haloes developed perfectly in this granite,
and having a nucleus of zircon less than 5 x 10-4 cms. in
diameter. The volume of zircon is 65 x 10-12 c.cs. and the mass
3 x 10-10 grm.; and if there was in this zircon 10-8 grm. radium
per gram (a quantity about five times the greatest amount
measured by Strutt), the mass of radium involved is 3 x 10-18
grm. From this and from the fact ascertained by Rutherford that
the number of alpha rays expelled by a gram of radium in one
second is 3.4 x 1010, we find that three rays are shot from the
nucleus in a year. If, now, geological time since the Devonian is
50 millions of years, then 150 millions of rays built up the
halo. If geological time since the Devonian is 400 millions of
years, then 1,200 millions of alpha rays are concerned in its
genesis. The number of ions involved, of course, greatly exceeds
these numbers. A single alpha ray fired from radium C will
produce 2.37 x 105 ions in air.
But haloes may be found quite clearly defined and fairly dark out
to the range of the emanation ray and derived from much less
quantities of radioactive materials. Thus a zircon nucleus with a
diameter of but 3.4 x 10-4 cms. formed a halo strongly darkened
within, and showing radium A and radium C as clear smoky rings.
Such a nucleus, on the assumption made above as to its radium
content, expels one ray in a year. But, again, haloes are
observed with less blackened pupils and with faint ring due to
radium C, formed round nuclei
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of rather less than 2 x 10-4 cms. diameter. Such nuclei would
expel one ray in five years. And even lesser nuclei will generate
in these old rocks haloes with their earlier characteristic
features clearly developed. In the case of the most minute
nuclei, if my assumption as to the uranium content is correct, an
alpha ray is expelled, probably, no oftener than once in a
century; and possibly at still longer intervals.
The equilibrium amount of radium contained in some nuclei may
amount to only a few atoms. Even in the case of the larger nuclei
and more perfectly developed haloes the quantity of radium
involved is many millions of times less than the least amount we
can recognise by any other means. But the delicacy of the
observation is not adequately set forth in this statement. We can
not only tell the nature of the radioactive family with which we
are dealing; but we can recognise the presence of some of its
constituent members. I may say that it is not probable the
zircons are richer in radium than I have assumed. My assumption
involves about 3 per cent. of uranium. I know of no analyses
ascribing so great an amount of uranium to zircon. The variety
cyrtolite has been found to contain half this amount, about. But
even if we doubled our estimate of radium content, the remarkable
nature of our conclusions is hardly lessened.
It may appear strange that the ever-interesting question of the
Earth's age should find elucidation from the
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study of haloes. Nevertheless the subjects are closely connected.
The circumstances are as follows. Geologists have estimated the
age of the Earth since denudation began, by measurements of the
integral effects of denudation. These methods agree in showing an
age of about rob years. On the other hand, measurements have been
made of the accumulation in minerals of radioactive _débris_--the
helium and lead--and results obtained which, although they do not
agree very well among themselves, are concordant in assigning a
very much greater age to the rocks. If the radioactive estimate
is correct, then we are now living in a time when the denudative
forces of the Earth are about eight or nine times as active as
they have been on the average over the past. Such a state of
things is absolutely unaccountable. And all the more
unaccountable because from all we know we would expect a somewhat
_lesser_ rate of solvent denudation as the world gets older and the
land gets more and more loaded with the washed-out materials of
the rocks.
Both the methods referred to of finding the age assume the
principle of uniformity. The geologist contends for uniformity
throughout the past physical history of the Earth. The physicist
claims the like for the change-rates of the radioactive elements.
Now the study of the rocks enables us to infer something as to
the past history of our Globe. Nothing is, on the other hand,
known respecting the origin of uranium or thorium--the parent
radioactive bodies. And while not questioning the law
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and regularity which undoubtedly prevail in the periods of the
members of the radioactive families, it appears to me that it is
allowable to ask if the change rate of uranium has been always
what we now believe it to be. This comes to much the same thing
as supposing that atoms possessing a faster change rate once were
associated with it which were capable of yielding both helium and
lead to the rocks. Such atoms might have been collateral in
origin with uranium from some antecedent element. Like helium,
lead may be a derivative from more than one sequence of
radioactive changes. In the present state of our knowledge the
possibilities are many. The rate of change is known to be
connected with the range of the alpha ray expelled by the
transforming element; and the conformity of the halo with our
existing knowledge of the ranges is reason for assuming that,
whatever the origin of the more active associate of uranium, this
passed through similar elemental changes in the progress of its
disintegration. There may, however, have been differences in the
ranges which the halo would not reveal. It is remarkable that
uranium at the present time is apparently responsible for two
alpha rays of very different ranges. If these proceed from
different elements, one should be faster in its change rate than
the other. Some guidance may yet be forthcoming from the study of
the more obscure problems of radioactivity.
Now it is not improbable that the halo may contribute directly to
this discussion. We can evidently attack
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the biotite with a known number of alpha rays and determine how
many are required to produce a certain intensity of darkening,
corresponding to that of a halo with a nucleus of measurable
dimensions. On certain assumptions, which are correct within
defined limits, we can calculate, as I have done above, the
number of rays concerned in forming the halo. In doing so we
assume some value for the age of the halo. Let us take the
maximum radioactive value. A halo originating in Devonian times
may attain a certain central blackening from the effects of, say,
rob rays. But now suppose we find that we cannot produce the same
degree of blackening with this number of rays applied in the
laboratory. What are we to conclude? I think there is only the
one conclusion open to us; that some other source of alpha rays,
or a faster rate of supply, existed in the past. And this
conclusion would explain the absence of haloes from the younger
rocks; which, in view of the vast range of effects possible in
the development of haloes, is, otherwise, not easy to account
for. It is apparent that the experiment on the biotite has a
direct bearing on the validity of the radioactive method of
estimating the age of the rocks. It is now being carried out by
Professor Rutherford under reliable conditions.
Finally, there is one very certain and valuable fact to be
learned from the halo. The halo has established the extreme
rarity of radioactivity as an atomic phenomenon. One and all of
the speculations as to
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the slow breakdown of the commoner elements may be dismissed. The
halo shows that the mica of the rocks is radioactively sensitive.
The fundamental criterion of radioactive change is the expulsion
of the alpha ray. The molecular system of the mica and of many
other minerals is unstable in presence of these rays, just as a
photographic plate is unstable in presence of light. Moreover,
the mineral integrates the radioactive effects in the same way as
a photographic salt integrates the effects of light. In both
cases the feeblest activities become ultimately apparent to our
inspection. We have seen that one ray in each year since the
Devonian period will build the fully formed halo: an object
unlike any other appearance in the rocks. And we have been able
to allocate all the haloes so far investigated to one or the
other of the known radioactive families. We are evidently
justified in the belief that had other elements been radioactive
we must either find characteristic haloes produced by them, or
else find a complete darkening of the mica. The feeblest alpha
rays emitted by the relatively enormous quantities of the
prevailing elements, acting over the whole duration of geological
time--and it must be remembered that the haloes we have been
studying are comparatively young--must have registered their
effects on the sensitive minerals. And thus we are safe in
concluding that the common elements, and, indeed, many which
would be called rare, are possessed of a degree of stability
which has preserved them un
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changed since the beginning of geological time. Each unaffected
flake of mica is, thus, unassailable proof of a fact which but
for the halo would, probably, have been for ever beyond our
cognisance.