result that the radio-active processes cannot in any way be changed by
the action of known forces. It may be recalled that the constant of
decay of the activity of a radio-active product has a definite fixed
value under all conditions. It is independent of the concentration of
the active matter, of the pressure, and of the nature of the gas in
which the substance is placed, and it is not affected by wide ranges of
temperature. The only observed exception is the product radium C. Its
value of λ increases with temperature to some extent at about 1000° C.,
but at 1200° C. returns nearly to the normal value. In the same way, it
has not been found possible to alter the rate of production of active
matter from the radio-elements. In addition, there is not a single
well-authenticated case where radio-activity has been altered or
destroyed in any active body or created in an inactive element.
Certain cases have been observed, which at first sight seem to indicate
a destruction of radio-activity. For example, the excited radio-activity
is removed from a platinum wire when heated above a red heat. It has
been shown, however, by Miss Gates (section 187) that the radio-activity
is not destroyed, but is deposited in unaltered amount on the colder
bodies surrounding it. Thorium oxide has been shown to lose to a large
extent its power of emanating by ignition to a white heat. But a close
examination shows that the emanation is still being produced at the same
rate, but is occluded in the compound.
The total radio-activity of a given mass of a radio-element, measured by
the peculiar radiations emitted, is a quantity which can neither be
increased nor diminished, although it may be manifested in a series of
products which are capable of separation from the radio-element. The
term “conservation of radio-activity” is thus a convenient expression of
the facts known at the present time. It is quite possible, however, that
further experiments at very high or very low temperatures may show that
the radio-activity does vary.
Although no difference has been observed in the radio-activity of
uranium over an interval of five years, it has been shown (section
261) that on theoretical grounds the radio-activity of a _given
quantity_ of a radio-element should decrease with the time. The change
will, however, be so slow in uranium, that probably millions of
years must elapse before a measurable change can take place, while the
total radio-activity of a given quantity of matter left to itself should
thus decrease, but it ought to be constant for a _constant mass_ of the
radio-element. It has already been pointed out (section 238) that the
activity of radium, measured by the α and β rays, will probably increase
for several hundred years after its separation. This is due to the
appearance of fresh products in the radium. Ultimately, however, the
activity must decrease according to an exponential law with the time,
falling to half value in about 1300 years.
The conservation of radio-activity applies not only to the radiations
taken as a whole, but also to each specific type of radiation. If the
emanation is removed from a radium compound, the amount of β radiation
of the radium at once commences to decrease, but this is compensated by
the appearance of β rays in the radiations from the vessel in which the
separated emanation is stored. At any time the sum total of the β
radiations from the radium and the emanation vessel is always the same
as that from the radium compound before the emanation was removed.
Similar results have also been found to hold for the γ rays. This was
tested by the writer in the following way. The emanation from some solid
radium bromide was released by heat, and condensed in a small glass tube
which was then sealed off. The radium so treated, and the emanation
tube, were placed together under an electroscope, with a screen of lead
1 cm. thick interposed in order to let through only the γ rays. The
experiments were continued over three weeks, but the sum total of the γ
rays from the radium and the emanation tube was, over the whole
interval, equal to that of the original radium. During this period the
amount of γ rays from the radium at first decreased to only a few per
cent. of the original value, and then slowly increased again, until at
the end of the three weeks it had nearly regained its original value,
before the emanation was removed. At the same time the amount of γ rays
from the emanation tube rose from zero to a maximum and then slowly
decreased again at the same rate as the decay of the activity of the
emanation in the tube. This result shows that the amount of γ rays from
radium was a constant quantity over the interval of observation,
although the amount of γ rays from the radium and emanation tube had
passed through a cycle of changes.
There is one interesting possibility in this connection that should be
borne in mind. The rays from the active substances carry off energy in a
very concentrated form, and this energy is dissipated by the absorption
of the rays in matter. The rays might be expected to cause a
disintegration of the atoms of inactive matter on which they fall and
thus give rise to a kind of radio-activity. This effect has been looked
for by several observers. Ramsay and W. T. Cooke[361] state that they
have noticed such an action, using about a decigram of radium as a
source of radiation. The radium, sealed in a glass vessel, was
surrounded by an external glass tube and exposed to the action of the β
and γ rays of radium for several weeks. The inside and outside of the
glass tube were found to be active, and the active matter was removed by
solution in water. The radio-activity observed was very minute,
corresponding to only about 1 milligram of uranium. The writer has, at
various times, tried experiments of this character but with negative
results. The greatest care is necessary in such experiments to ensure
that the radio-activity is not due to other causes besides the rays from
the radium. This care is especially necessary in laboratories where
considerable quantities of the radium emanation have been allowed to
escape into the air. The surface of every substance becomes coated with
the slow transformation products of radium, viz. radium D, E, and F. The
activity communicated in this way to originally inactive matter is often
considerable. This infection by the radium emanation extends throughout
the whole laboratory, on account of the distribution of the emanation by
convection and diffusion. For example, Eve[362] found that every
substance which he examined in the laboratory of the writer showed much
greater activity than the normal. In this case the radium had been in
use in the building for about two years.
=265. Loss of weight of the radio-elements.= Since the radio-elements
are continually throwing off α particles atomic in size, an active
substance, enclosed in a vessel sufficiently thin to allow the α
particles to escape, must gradually lose in weight. This loss of weight
will be small under ordinary conditions, since the greater proportion of
the α rays produced are absorbed in the mass of the substance. If a very
thin layer of a radium compound were spread on a very thin sheet of
substance, which did not appreciably absorb the α particles, a loss of
weight due to the expulsion of α particles might be detectable. Since
_e_/_m_ = 6 × 10³ for the α particle and _e_ = 1·1 × 10⁻²⁰
electromagnetic units and 2·5 × 10¹¹ α particles are expelled per
second per gram of radium, the proportion of the mass expelled is 4·8 ×
10⁻¹³ per second and 10⁻⁵ per year. There is one condition, however,
under which the radium should lose in weight fairly rapidly. If a
current of air is slowly passed over a radium solution, the emanation
produced would be removed as fast as it was formed. Since the atom of
the emanation has a mass probably not much smaller than the radium atom,
the fraction of the mass removed per year should be nearly equal to the
fraction of the radium which changes per year, _i.e._ one gram of radium
should diminish in weight about half a milligram (section 261) per year.
If it is supposed that the β particles have weight, the loss of weight
due to their expulsion is very small compared with that due to the
emission of α particles. The writer has shown (section 253) that about
7 × 10¹⁰ β particles are projected per second from 1 gram of radium.
The consequent loss of weight would only be about 10⁻⁹ grams per year.
Except under very special experimental conditions, it would thus be
difficult to detect the loss of weight of radium due to the expulsion of
β particles from its mass. There is, however, a possibility that radium
might change in weight even though none of the radio-active products
were allowed to escape. For example, if the view is taken that
gravitation is the result of forces having their origin in the atom, it
is possible that, if the atom were disintegrated, the weight of the
parts might not be equal to that of the original atom.
A large number of experiments have been made to see if radium
preparations, kept in a sealed tube, alter in weight. With the small
quantities of radium available to the experimenter, no difference of
weight of radium preparations with time has yet been established with
certainty. Heydweiller stated that he had observed a loss of weight of
radium and Dorn also obtained a slight indication of change in weight.
These results have not, however, been confirmed. Forch, later, was
unable to observe any appreciable change.
J. J. Thomson[363] has made experiments to see if the ratio of weight to
mass for radium is the same as for inactive matter. We have seen in
section 48 that a charge in motion possesses an apparent mass which is
constant for slow speeds but increases as the speed of light is
approached. Now radium emits some electrons at a velocity comparable
with the velocity of light, and presumably these electrons were in rapid
motion in the atom before their expulsion. It might thus be possible
that the ratio for radium would differ from that for ordinary matter.
The pendulum method was used, and the radium was enclosed in a small
light tube suspended by a silk fibre. Within the limit of experimental
error the ratio of weight to mass was found to be the same as for
ordinary matter, so that we may conclude that the number of electrons
moving with a velocity approaching that of light is small compared with
the total number present.
=266. Total emission of energy from the radio-element.= It has been
shown that 1 gram of radium emits energy at the rate of 100
gram-calories per hour or 876,000 gram-calories per year. If 1 gram of
radium in radio-active equilibrium be set apart, its radio-activity and
consequent heat emission is given at a time _t_ by
$$ qe^{–λt} $$,
where λ is the constant of decay of activity of radium and of the
initial heating effect; the total heat emission from 1 gram of radium is
given by
$$ \int₀^{\infty} qe^{–λt} dt = \frac {q} {λ} $$ .
Now on the estimate of the life of radium given in section 261 the value
of λ is ¹⁄₁₈₅₀ when 1 year is taken as the unit of time. The total heat
emission from 1 gram of radium during its life is thus 1·6 × 10⁹
gram-calories. The heat emitted in the union of hydrogen and oxygen to
form 1 gram of water is about 4 × 10³ gram-calories, and in this
reaction more heat is given out for equal weights than in any other
chemical reaction known. It is thus seen that the total energy emitted
from 1 gram of radium during its changes is about one million times
greater than in any known molecular change. That matter is able, under
special conditions, to emit an enormous amount of energy, is well
exemplified by the case of the radium emanation. Calculations of the
amount of this energy have already been given in section 249.
Since the other radio-elements only differ from radium in the slowness
of their change, the total heat emission from uranium and thorium must
be of a similar high order of magnitude. There is thus reason to believe
that there is an enormous store of latent energy resident in the atoms
of the radio-elements. This store of energy could not have been
recognized if the atoms had not been undergoing a slow process of
disintegration. The energy emitted in radio-active changes is derived
from the internal energy of the atoms. The emission of this energy does
not disobey the law of the conservation of energy, for it is only
necessary to suppose that, when the radio-active changes have ceased,
the energy stored up in the atoms of the final products is less than
that of the original atoms of the radio-elements. The difference between
the energy originally possessed by the matter which has undergone the
change, and the final inactive products which arise, is a measure of the
total amount of energy released.
There seems to be every reason to suppose that the atomic energy of all
the elements is of a similar high order of magnitude. With the exception
of their high atomic weights, the radio-elements do not possess any
special chemical characteristics which differentiate them from the
inactive elements. The existence of a latent store of energy in the
atoms is a necessary consequence of the modern view developed by J. J.
Thomson, Larmor, and Lorentz, of regarding the atom as a complicated
structure consisting of charged parts in rapid oscillatory or orbital
motion in regard to one another. The energy may be partly kinetic and
partly potential, but the mere concentration of the charged particles,
which probably constitute the atom, in itself implies a large store of
energy in the atom, in comparison with which the energy emitted during
the changes of radium is insignificant.
The existence of this store of latent energy does not ordinarily
manifest itself, since the atoms cannot be broken up into simpler forms
by the physical or chemical agencies at our disposal. Its existence at
once explains the failure of chemistry to transform the atoms, and also
accounts for the rate of change of the radio-active processes being
independent of all external agencies. It has not so far been found
possible to alter in any way the rate of emission of energy from the
radio-elements. If it should ever be found possible to control at will
the rate of disintegration of the radio-elements, an enormous amount of
energy could be obtained from a small quantity of matter.
=267. Production of helium from radium and the radium emanation.= Since
the final products, resulting from a disintegration of the
radio-elements, are not radio-active, they should in the course of
geologic ages collect in some quantity, and should always be found
associated with the radio-elements. Now the inactive products resulting
from the radio-active changes are the α particles expelled at each
stage, and the final inactive product or products which remain, when the
process of disintegration can no longer be traced by the property of
radio-activity.
Pitchblende, in which the radio-elements are mostly found, contains in
small quantity a large proportion of all the known elements. In
searching for a possible disintegration product common to all the
radio-elements, the presence of helium in the radio-active minerals is
noteworthy; for helium is only found in the radio-active minerals, and
is an invariable companion of the radio-elements. Moreover, the presence
in minerals of a light, inert gas like helium had always been a matter
of surprise. The production by radium and thorium of the radio-active
emanations, which behave like chemically inert gases of the helium-argon
family, suggested the possibility that one of the final inactive
products of the disintegration of the radio-elements might prove to be a
chemically inert gas. The later discovery of the material nature of the
α rays added weight to the suggestion; for the measurement of the ratio
_e_/_m_ of the α particle indicated that if the α particle consisted of
any known kind of matter, it must either be hydrogen or helium. For
these reasons, it was suggested in 1902 by Rutherford and Soddy[364]
that helium might be a product of the disintegration of the
radio-elements.
Sir William Ramsay and Mr Soddy in 1903 undertook an investigation of
the radium emanation, with the purpose of seeing if it were possible to
obtain any spectroscopic evidence of the presence of a new substance.
First of all, they exposed the emanation to very drastic treatment
(section 158), and confirmed and extended the results previously noted
by Rutherford and Soddy that the emanation behaved like a chemically
inert gas, and in this respect possessed properties analogous to the
gases of the helium-argon group.
On obtaining 30 milligrams of pure radium bromide (prepared about three
months previously) Ramsay and Soddy[365] examined the gases, liberated
by solution of the radium bromide in water, for the presence of helium.
A considerable quantity of hydrogen and oxygen was released by the
solution (see section 124). The hydrogen and oxygen were removed by
passing the liberated gases over a red-hot spiral of partially
oxidized copper-wire and the resulting water vapour was absorbed in
a phosphorus pentoxide tube.
The gas was then passed into a small vacuum tube which was in connection
with a small =U= tube. By placing the =U= tube in liquid air, most of
the emanation present was condensed, and also most of the CO₂ present in
the gas. On examining the spectrum of the gas in the vacuum tube, the
characteristic line _D₃_ of helium was observed.
This experiment was repeated with 30 milligrams of radium bromide about
four months old, lent for the purpose by the writer. The emanation and
CO₂ were removed by passing them through a =U= tube immersed in liquid
air. A practically complete spectrum of helium was observed, including
the lines of wave-lengths 6677, 5876, 5016, 4972, 4713 and 4472. There
were also present three other lines of wave-lengths about 6180, 5695,
5455 which have not yet been identified.
In later experiments, the emanation from 50 milligrams of the radium
bromide was conveyed with oxygen into a small =U= tube, cooled in liquid
air, in which the emanation was condensed. Fresh oxygen was added, and
the =U= tube again pumped out. The small vacuum tube, connected with the
=U= tube, showed at first no helium lines when the liquid air was
removed. The spectrum obtained was a new one, and Ramsay and Soddy
considered it to be probably that of the emanation itself. After
allowing the emanation tube to stand for four days, the helium spectrum
appeared with all the characteristic lines, and in addition, three new
lines present in the helium obtained by solution of the radium. These
results have since been confirmed. The experiments, which have led to
such striking and important results, were by no means easy of
performance, for the quantity of helium and of emanation released from
50 mgrs. of radium bromide is extremely small. It was necessary, in all
cases, to remove almost completely the other gases, which were present
in sufficient quantity to mask the spectrum of the substance under
examination. The success of the experiments has been largely due to the
application, to this investigation, of the refined methods of gas
analysis, previously employed by Sir William Ramsay with so much skill
in the separation of the rare gases xenon and krypton, which exist in
minute proportions in the atmosphere. The fact that the helium spectrum
was not present at first, but appeared _after_ the emanation had
remained in the tube for some days, shows that the helium must have been
produced from the emanation. The emanation cannot be helium itself, for,
in the first place, helium is not radio-active, and in the second place,
the helium spectrum was not present at first, when the quantity of
emanation in the tube was at its maximum. Moreover, the diffusion
experiments, already discussed, point to the conclusion that the
emanation is of high molecular weight. There can thus be no doubt that
the helium is derived from the emanation of radium in consequence of
changes of some kind occurring in it.
These results were confirmed later by other observers. Curie and
Dewar[366] performed the following experiment: A weight of about ·42 gr.
of radium bromide was placed in a quartz tube, and the tube exhausted
until no further gas came off. The radium was then heated to fusion,
about 2·6 c.c. of gas being liberated in the process. The tube was then
sealed, and some weeks afterwards the spectrum of the gas liberated in
the tube by the radium was examined by Deslandres and found to give the
entire spectrum of helium. The gas, liberated during the initial heating
of the radium, was collected and found to contain a large amount of
emanation, although the gas had been passed through two tubes immersed
in liquid air. The tube containing these gases was very luminous and
rapidly turned violet, while more than half of the gases was absorbed.
The spectrum of the phosphorescent light was found to be discontinuous,
consisting of three nitrogen bands. No sign of the helium spectrum was
observed, although helium must have been present.
Himstedt and Meyer[367] placed 50 mgrs. of radium bromide in a =U= tube
connected with a small vacuum tube. The tube was carefully exhausted and
then sealed off. The spectrum of hydrogen and carbon dioxide alone was
observed for three months, but after four months the red, yellow, green
and blue lines of the helium spectrum were visible. The slow appearance
of the helium spectrum was probably due to the presence in the tube of a
considerable quantity of hydrogen. In another experiment, some radium
sulphate which had been heated to a bright red heat in a quartz tube was
connected with a small vacuum tube. After three weeks, some of the lines
of helium were clearly seen, and increased in brightness with time.
=268. Connection between helium and the α particles=. The appearance of
helium in a tube containing the radium emanation may indicate either
that the helium is one of the final products, which appear at the end of
the series of radio-active changes, or that the helium is in reality the
expelled α particle. The evidence at present points to the latter as
being the more probable explanation. In the first place, the emanation
diffuses like a gas of heavy molecular weight, and it appears probable
that after the expulsion of a few α particles, the atomic weight of the
final product is comparable with that of the emanation. On the other
hand, the value of _e_/_m_ determined for the projected α particle
points to the conclusion that, if it consists of any known kind of
matter, it is either hydrogen or helium.
There has been a tendency to assume that the helium produced from the
radium emanation is the last transformation product of that substance.
The evidence, however, does not support this view. We have seen that the
emanation, after the initial rapid changes, is transformed very slowly.
If the helium were the final product, the amount present in the
emanation tube after a few days or weeks would be insignificant, since
the product radium D intervenes, which takes 40 years to be half
transformed. Since the helium cannot be the final product of the series
of changes, and since all the other products are radio-active, and
almost certainly of high atomic weight, it is difficult to see what
position the helium atom occupies in the scheme of transformation,
unless it be the α particle expelled during the successive changes.
It is a matter of great difficulty to settle definitely whether the α
particle is a projected helium atom or not. On account of the very small
deflection of the α rays in an electric field, and the complex nature of
the α radiation from radium, an accurate determination of the value
_e_/_m_ for the α particle is beset with difficulties.
It may be possible to settle the question by accurate measurements of
the volume of gas in a tube, filled originally with the radium
emanation. Since the emanation itself, and two of the rapidly changing
products which result from it, emit α particles, the final volume of the
α particles, if they can exist in the gaseous state, would be three
times the volume of the emanation. Ramsay and Soddy (section 172) have
made experiments of this kind, but the results obtained were very
contradictory, depending upon the kind of glass employed. In one case,
the volume of the residual gases shrank almost to zero, in another the
initial volume increased to about ten times its initial value. In the
latter experiment a brilliant spectrum of helium was observed in the
residual gas. This difference of behaviour is probably due to different
degrees of absorption of helium by the glass tubes.
If the α particles are helium atoms, we may expect that a large
proportion of the helium, which is produced in a tube containing the
radium emanation, will be buried in the wall of the glass tube; for the
α particles are projected with sufficient velocity to penetrate some
distance into the glass. This helium may either remain in the glass, or
in some cases rapidly diffuse out again. In any case, a fraction of the
helium would be liberated when an intense electric discharge is passed
through the tube. Ramsay and Soddy have in some instances observed that
a slight amount of helium is liberated on heating the walls of the tube
in which the emanation had been stored for some time.
The volume of helium produced per year by 1 gram of radium can easily be
calculated on the assumption that the α particle is in reality a helium
atom.
It has been shown that 2·5 × 10¹¹ α particles are projected per second
from 1 gram of radium. Since there are 3·6 × 10¹⁹ molecules in one cubic
centimetre of any gas at standard pressure and temperature, the volume
of the α particles released per second is 7 × 10⁻⁹ c.c. and per year
0·24 c.c. It has already been pointed out that, on this hypothesis, the
volume of helium released by the emanation is three times the volume of
the latter. The amount of helium to be obtained from the emanation
released from 1 gram of radium in radio-active equilibrium is thus about
3 cubic mms.
Ramsay and Soddy have tried to estimate experimentally the probable
volume of helium produced per second by one gram of radium. The helium,
obtained from 50 mgrs. of radium bromide, which had been kept in
solution in a closed vessel for 60 days, was introduced into a vacuum
tube. Another similar tube was placed in series with it, and the amount
of the helium in the latter adjusted until on passing a discharge
through the two tubes in series the helium lines in each tube were of
about the same brightness. In this way they calculated that the amount
of helium present was 0·1 cubic mm. On this estimate, the amount of
helium produced per year per gram of radium is about 20 cubic mms. We
have seen that the calculated amount is about 240 cubic mms., on the
assumption that the α particle is a helium atom. Ramsay and Soddy
consider that the presence of argon in one of the tubes may have
seriously interfered with the correctness of the estimation. On account
of the great uncertainty attaching to estimates of the above character,
the value deduced by Ramsay and Soddy does not exclude the probability
that the calculated volume may be of the right order of magnitude.
In order to explain the presence of helium in radium on ordinary
chemical lines, it has been suggested that radium is not a true element,
but a molecular compound of helium with some substance known or unknown.
The helium compound gradually breaks down, giving rise to the helium
observed. It is at once obvious that this postulated helium compound is
of a character entirely different from that of any other compound
previously observed in chemistry. Weight for weight, it emits during its
change an amount of energy at least one million times greater than any
molecular compound known (see section 249). In addition, it must be
supposed that the rate of breaking up of the helium compound is
independent of great ranges of temperature—a result never before
observed in any molecular change. The helium compound in its breaking up
must give rise to the peculiar radiations and also pass through the
successive radio-active changes observed in radium.
Thus in order to explain the production of helium and radio-activity on
this view, a unique kind of molecule must be postulated—a molecule, in
fact, which is endowed with every single property which on the
disintegration theory is ascribed to the atom of the radio-elements. On
the other hand, radium as far as it has been examined, has fulfilled
every test required for an element. It has a well-marked and
characteristic spectrum, and there is no reason to suppose that it is
not an element in the ordinarily accepted sense of the term.
On the theory that the radio-elements are undergoing atomic
disintegration, the helium must be considered to be a constituent of the
radium atom, or, in other words, the radium atom is built up of parts,
one of which, at least, is the atom of helium. The theory that the heavy
atoms are all built up of some simple fundamental unit of matter or
protyle has been advanced at various times by many prominent chemists
and physicists. Prout’s hypothesis that all elements are built up out of
hydrogen is an example of this point of view of regarding the subject.
On the disintegration theory, the changes occurring in the radio-atoms
involve an actual transformation of the atoms through successive
changes. This change is so slow in uranium and thorium that at least a
million years would be required before the amount of change could be
measured by the balance. In radium it is a million times faster, but
even in this case it is doubtful whether any appreciable change would
have been observed by ordinary chemical methods for many years had not
the possibility of such a change been suggested from other lines of
evidence.
The similarity of the α particles from the different radio-elements
indicates that they consist of expelled particles of the same kind. On
this view, helium should be produced by each of the radio-elements. Its
presence in minerals containing thorium, for example in monazite sand
and the Ceylon mineral described by Ramsay, indicates that helium may be
a product of thorium as well as of radium. Strutt[368] has recently
suggested that most of the helium observed in radio-active minerals may
be a decomposition product of thorium rather than of uranium and radium;
for he finds that minerals rich in helium always contain thorium, while
many uranium minerals nearly free from thorium contain little helium.
The evidence in support of this view is, however, not altogether
satisfactory, for some of the uranium minerals in question are secondary
uranium minerals (see Appendix B), deposited by the action of water or
other agencies at a comparatively late date, and are also, in many
cases, highly emanating, and consequently could not be expected to
retain more than a fraction of the helium produced in them.
Taking the view that the α particles are projected helium atoms, we must
regard the atoms of the radio-elements as compounds of some known or
unknown substance with helium. These compounds break up spontaneously,
and at a very slow rate even in the case of radium. The disintegration
takes place in successive stages, and at most of the stages a helium
atom is projected with great velocity. This disintegration is
accompanied by an enormous emission of energy. The liberation of such a
large amount of energy in the radio-active changes at once explains the
constancy of the rate of change under the action of any of the physical
and chemical agencies at our command. On this view, uranium, thorium and
radium are in reality compounds of helium. The helium, however, is held
in such strong combination that the compound cannot be broken up by
chemical or physical forces, and, in consequence, these bodies behave as
chemical elements in the ordinary accepted chemical sense.
It appears not unlikely that many of the so-called chemical elements may
prove to be compounds of helium, or, in other words, that the helium
atom is one of the secondary units with which the heavier atoms are
built up. In this connection it is of interest to note that many of the
elements differ in their atomic weight by four—the atomic weight of
helium.
If the α particle is a helium atom, at least three α particles must be
expelled from uranium (238·5) to reduce its atomic weight to that of
radium (225). It is known that five α particles are expelled from radium
during its successive transformations. This would make the atomic weight
of the final residue 225 − 20 = 205. This is very nearly the atomic
weight of lead, 206·5. I have, for some time, considered it probable
that lead is the end or final product of radium. The same suggestion has
recently been made by Boltwood[369]. This point of view is supported by
the fact that lead is always found in small quantity in all uranium
minerals, and that the relative proportions of lead and helium in the
radio-active minerals are about the same as would be expected if lead
and helium were both decomposition products of radium. Dr Boltwood has
drawn my attention to the fact that the proportion of lead in many
radio-active minerals varies with the content of helium. A mineral rich
in helium in nearly all cases contains more lead than a mineral poor in
helium. This cannot be considered, at present, more than a speculation,
but the facts as they stand are very suggestive.
=269. Age of radio-active minerals.= Helium is only found in the
radio-active minerals, and this fact, taken in conjunction with the
liberation of helium by radium, indicates that the helium must have been
produced as a result of the transformation of radium and the other
radio-active substances contained in the minerals. Now in a mineral
about half the helium is, in many cases, released by heat and the
residue by solution. It seems probable that the helium produced
throughout the mass of the mineral is mechanically imprisoned in it.
Moss[370] found that, by grinding pitchblende in vacuo, helium is
evolved, apparently showing that the helium exists in cavities of the
mineral. Travers[371] has suggested that, since helium is liberated on
heating, the effect may be due to the heat generated by grinding. The
escape of the helium from the heated mineral is probably connected with
the fact observed by Jaquerod[372] that helium passes through the walls
of a quartz tube, heated above 500° C. The substance of the mineral
probably possesses a similar property. Travers considers that helium is
present in the mineral in a state of supersaturated solid solution, but
the facts are equally well explained by assuming that the helium is
mechanically imprisoned in the mass of the mineral.
The sudden rise of temperature observed in the mineral fergusonite, at
the time the helium is released, has been found to have nothing to do
with the presence of helium, for it also takes place in minerals not
containing helium. The old view that helium was in a state of chemical
combination with the mineral must be abandoned in the light of these
more recent experiments.
Since the helium is only released from some minerals by the action of
high temperatures and solution, it appears probable that a large
proportion of the helium found in the minerals is unable to escape under
normal conditions. Thus if the rate of production of helium by the
radio-active substance were definitely known, it should be possible to
calculate the age of the mineral by observing the volume of helium
liberated from it by solution.
In the absence of such definite information, an approximate calculation
will be made to indicate the order of magnitude of the time that has
elapsed since the mineral was formed or was at a temperature low enough
to prevent the escape of the helium.
Let us take, for example, the mineral fergusonite, which was found by
Ramsay and Travers[373] to evolve 1·81 c.c. of helium. The fergusonite
contained about 7 per cent. of uranium. Now uranium in old minerals
probably contains about 8 × 10⁻⁷ of its weight of radium (see section
262). One gram of the mineral thus contained about 5·6 × 10⁻⁸ grams of
radium. Now if the α particle is helium, it has been shown that 1 gram
of radium produces 0·24 c.c. of helium per year. The volume of helium
produced per year in 1 gram of fergusonite is thus 1·3 × 10⁻⁸ c.c.
Assuming that the rate of production of helium has been uniform, the
time required to produce 1·81 c.c. per gram is about 140 million years.
If the calculated rate of production of helium by radium is an
over-estimate, the time is correspondingly lengthened.
I think that, when the constants required for these calculations are
more definitely fixed, this method will probably give fairly trustworthy
information as to the probable age of some of the radio-active minerals
of the earth’s crust, and indirectly as to the age of the strata in
which they are found.
In this connection it is of interest to note that Ramsay[374] found that
a Ceylon mineral, thorianite, contained as much as 9·5 c.c. of helium
per gram. According to the analysis by Dunstan, this mineral contains
about 76 per cent. of thorium and 12 per cent. of uranium. The unusually
large amount of helium evolved from this mineral would indicate that it
was formed at an earlier date than the fergusonite previously
considered.
=270. Possible causes of disintegration.= In order to explain the
phenomena of radio-activity, it has been supposed that a certain small
fraction of the radio-atoms undergoes disintegration per second, but no
assumptions have been made as to the cause which produces the
instability and consequent disintegration. The instability of the atoms
may be supposed to be brought about either by the action of external
forces or by that of forces inherent in the atoms themselves. It is
conceivable, for example, that the application of some slight external
force might cause instability and consequent disintegration, accompanied
by the liberation of a large amount of energy, on the same principle
that a detonator is necessary to start some explosives. It has been
shown that the number of atoms of any radio-active product which break
up per second is always proportional to the number present. This law of
change does not throw any light on the question, for it would be
expected equally on either hypothesis. It has not been found possible to
alter the rate of change of any product by the application of any known
physical or chemical forces, unless possibly it is assumed that the
force of gravitation which is not under our control may influence in
some way the stability of the radio-atoms.
It seems likely therefore that the cause of the disruption of the atoms
of the radio-elements and their products resides in the atoms
themselves. According to the modern views of the constitution of the
atom, it is not so much a matter of surprise that some atoms
disintegrate as that the atoms of the elements are so permanent as they
appear to be. In accordance with the hypothesis of J. J. Thomson, it may
be supposed that the atoms consist of a number of small positively and
negatively charged particles in rapid internal movement, and held in
equilibrium by their mutual forces. In a complex atom, where the
possible variations in the relative motion of the parts are very great,
the atom may arrive at such a phase that one part acquires sufficient
kinetic energy to escape from the system, or that the constraining
forces are momentarily neutralised, so that the part escapes from the
system with the velocity possessed by it at the instant of its release.
Sir Oliver Lodge[375] has advanced the view that the instability of the
atom may be a result of radiation of energy by the atom. Larmor has
shown that an electron, subject to acceleration, radiates energy at a
rate proportional to the square of its acceleration. An electron moving
uniformly in a straight line does not radiate energy, but an electron,
constrained to move in a circular orbit with constant velocity, is a
powerful radiator, for in such a case the electron is continuously
accelerated towards the centre. Lodge considered the simple case of a
negatively charged electron revolving round an atom of mass relatively
large but having an equal positive charge and held in equilibrium by
electrical forces. This system will radiate energy, and, since the
radiation of energy is equivalent to motion in a resisting medium, the
particle tends to move towards the centre, and its speed consequently
increases. The rate of radiation of energy will increase rapidly with
the speed of the electron. When the speed of the electron becomes very
nearly equal to the velocity of light, according to Lodge, another
effect supervenes. It has been shown (section 82) that the apparent mass
of an electron increases very rapidly as the speed of light is
approached, and is theoretically infinite at the speed of light. There
will be at this stage a sudden increase of the mass of the revolving
atom, and, on the supposition that this stage can be reached, a
consequent disturbance of the balance of forces holding the system
together. Lodge considers it probable that, under these conditions, the
parts of the system will break asunder and escape from the sphere of one
another’s influence.
It seems probable that the primary cause of the disintegration of the
atom must be looked for in the loss of energy of the atomic system due
to electromagnetic radiation (section 52). Larmor[376] has shown that
the condition to be fulfilled in order that a system of rapidly moving
electrons may persist without loss of energy is that the vector sum of
the accelerations towards the centre should be permanently zero. While a
single electron moving in a circular orbit is a powerful radiator of
energy, it is remarkable how rapidly the radiation of energy diminishes
if several electrons are revolving in a ring. This has recently been
shown by J. J. Thomson[377], who examined mathematically the case of a
system of negatively electrified corpuscles, situated at equal intervals
round the circumference of a circle, and rotating in one plane with
uniform velocity round its centre. For example, he found that the
radiation from a group of six particles moving with a velocity of ⅒
of the velocity of light is less than one-millionth part of the
radiation from a single particle describing the same orbit with the same
velocity. When the velocity is ¹⁄₁₀₀ of that of light the amount of
radiation is only 10⁻¹⁶ that of a single particle moving with the same
velocity in the same orbit.
Results of this kind indicate that an atom consisting of a large number
of revolving electrons may radiate energy extremely slowly, and yet,
finally, this minute but continuous drain of energy from the atom must
result either in a rearrangement of its component parts into a new
system, or of an expulsion of electrons or groups of electrons from the
atom.
Simple models of atoms to imitate the behaviour of polonium in shooting
out α particles, and of radium in shooting out β particles have been
discussed by Lord Kelvin[378]. It is possible to devise certain stable
arrangements of the positively and negatively electrified particles,
supposed to constitute an atom, which, on the application of some
disturbing force, break up with the expulsion of a part of the system
with great velocity.
J. J. Thomson[379] has mathematically investigated the possible stable
arrangements of a number of electrons moving about in a sphere of
uniform positive electrification. The properties of such a model atom
are very striking, and indirectly suggest a possible explanation of the
periodic law in chemistry. He has shown that the electrons, if in one
plane, arrange themselves in a number of concentric rings; and
generally, if they are not constrained to move in one plane, in a number
of concentric shells like the coats of an onion.
The mathematical problem is much simplified if the electrons are
supposed to rotate in rings in one plane, the electrons in each ring
being arranged at equal angular intervals. The ways in which the number
of electrons group themselves, for numbers ranging from 60 to 5 at
intervals of 5, are shown in the following table:—
Number 60 55 50 45 40 35
of
electrons
Number in 20 19 18 17 16 16
successive
rings
16 16 15 14 13 12
13 12 11 10 8 6
8 7 5 4 3 1
3 1 1
Number 30 25 20 15 10 5
of
electrons
Number in 15 13 12 10 8 5
successive
rings
10 9 7 5 2
5 3 1
In the next table is given the possible series of arrangements of
electrons which can have an outer ring of 20:—
Number 59 60 61 62 63 64 65 66 67
of
electrons
Number in 20 20 20 20 20 20 20 20 20
successive
rings
16 16 16 17 17 17 17 17 17
13 13 13 13 13 13 14 14 15
8 8 9 9 10 10 10 10 10
2 3 3 3 3 4 4 5 5
The smallest number of electrons which can have an outer ring of 20 is
59, while 67 is the greatest.
The various arrangements of electrons can be classified into families,
in which the groupings of the electrons have certain features in common.
Thus the group of 60 electrons consists of the same arrangement of
electrons as the group of 40 with the addition of an outer ring of 20
electrons; the group of 40 is the same as the group of 24 with an
additional ring outside; and the group of 24 in turn is the same as the
group of 11 with an extra ring. A series of model atoms may be formed in
this way, in which each atom is derived from the preceding member by an
additional ring of electrons. Such atoms would be expected to possess
many properties in common, and would correspond to the elements in the
same vertical column of the periodic table of Mendeléef.
Different arrangements of electrons vary widely in stability. Some may
acquire an extra electron or two and yet remain stable, others readily
lose an electron without disturbing their stability. The former would
correspond to an electro-negative atom, the latter to an
electro-positive.
Certain arrangements of electrons are stable if the electrons move with
an angular velocity greater than a certain value, but become unstable
when the velocity falls below this value. Four electrons in motion, for
example, are stable in one plane, but when the velocity falls below a
certain critical value, the system is unstable, and the electrons tend
to arrange themselves at the corners of a regular tetrahedron. J. J.
Thomson (_loc. cit._) applies this property to explain why an atom of
radio-active matter breaks up, as follows:—
“Consider now the properties of an atom containing a system of
corpuscles (electrons) of this kind. Suppose the corpuscles were
originally moving with velocities far exceeding the critical velocity;
in consequence of the radiation from the moving corpuscles, their
velocity will slowly—very slowly—diminish; when, after a long interval,
the velocity reaches the critical velocity, there will be what is
equivalent to an explosion of the corpuscles, the corpuscles will move
far away from their original position, their potential energy will
decrease, while their kinetic energy will increase. The kinetic energy
gained in this way might be sufficient to carry the system out of the
atom, and we should have, as in the case of radium, a part of the atom
shot off. In consequence of the very slow dissipation of energy by
radiation the life of the atom would be very long. We have taken the
case of the four corpuscles as the type of a system which, like a top,
requires for its stability a certain amount of rotation. Any system
possessing this property would, in consequence of the gradual
dissipation of energy by radiation, give to the atom containing it
radio-active properties similar to those conferred by the four
corpuscles.”
=271. Heat of the sun and earth.= It was pointed out by Rutherford and
Soddy[380] that the maintenance of the sun’s heat for long intervals of
time did not present any fundamental difficulty if a process of
disintegration, such as occurs in the radio-elements, were supposed to
be taking place in the sun. In a letter to _Nature_ (July 9, 1903) W. E.
Wilson showed that the presence of 3·6 grams of radium in each cubic
metre of the sun’s mass was sufficient to account for the present rate
of emission of energy by the sun. This calculation was based on the
estimate of Curie and Laborde that 1 gram of radium emits 100
gram-calories per hour, and on the observation of Langley that each
square centimetre of the sun’s surface emits 8·28 × 10⁶ gram-calories
per hour. Since the average density of the sun is 1·44, the presence of
radium in the sun, to the extent of 2·5 parts by weight in a million,
would account for its present rate of emission of energy.
An examination of the spectrum of the sun has not so far revealed any of
the radium lines. It is known, however, from spectroscopic evidence that
helium is present, and this indirectly suggests the existence of
radio-active matter also. It can readily be shown[381] that the absence
of penetrating rays from the sun at the surface of the earth does not
imply that the radio-elements are not present in the sun. Even if the
sun were composed of pure radium, it would hardly be expected that the γ
rays emitted would be appreciable at the surface of the earth, since the
rays would be almost completely absorbed in passing through the
atmosphere, which corresponds to a thickness of 76 centimetres of
mercury.
In the Appendix E of Thomson and Tait’s _Natural Philosophy_, Lord
Kelvin has calculated the energy lost in the concentration of the sun
from a condition of infinite dispersion, and concludes that it seems “on
the whole probable that the sun has not illuminated the earth for
100,000,000 years and almost certain that he has not done so for
500,000,000 years. As for the future we may say, with equal certainty,
that inhabitants of the earth cannot continue to enjoy the light and
heat essential to their life for many million years longer, unless
sources now unknown to us are prepared in the great storehouses of
creation.”
The discovery that a small mass of a substance like radium can emit
spontaneously an enormous quantity of heat renders it possible that this
estimate of the age of the sun’s heat may be much increased. In a letter
to _Nature_ (Sept. 24, 1903) G. H. Darwin drew attention to this
probability, and at the same time pointed out that, on Kelvin’s
hypotheses, his estimate of the duration of the sun’s heat was probably
much too high, and stated that, “The lost energy of the sun, supposed to
be a homogeneous sphere of mass _M_ and radius _a_, is (⅗)μ_M²_/_a_
where μ is the constant of gravitation. On introducing numerical values
for the symbols in this formula, I find the lost energy to be 2·7 × 10⁷
_M_ calories where _M_ is expressed in grams. If we adopt Langley’s
value of the solar constant, this heat suffices to give a supply for 12
million years. Lord Kelvin used Pouillet’s value for that constant, but
if he had been able to use Langley’s, his 100 million would have been
reduced to 60 million. The discrepancy between my results of 12 million
and his of 60 million is explained by a conjectural augmentation of the
lost energy to allow for the concentration of the solar mass towards its
central parts.” Now it has been shown (section 266) that one gram of
radium emits during its life an amount of heat corresponding to 1·6 ×
10⁹ gram-calories. It has also been pointed out that there is every
reason to suppose that a similar amount of energy is resident in the
chemical atoms of the inactive elements. It is not improbable that, at
the enormous temperature of the sun, the breaking up of the elements
into simpler forms may be taking place at a more rapid rate than on the
earth. If the energy resident in the atoms of the elements is thus
available, the time during which the sun may continue to emit heat at
the present rate may be at least 50 times longer than the value computed
from dynamical data.
Similar considerations apply to the question of the age of the earth. A
full discussion of the probable age of the earth, computed from its
secular cooling from a molten mass, is given by Lord Kelvin in Appendix
D of Thomson and Tait’s _Natural Philosophy_. He has there shown that
about 100 million years after the earth was a molten mass, the gradual
cooling due to radiation from its surface would account for the average
temperature gradient of ¹⁄₅₀° F. per foot, observed to-day near the
earth’s surface.
Some considerations will now be discussed which point to the probability
that the present temperature gradient observed in the earth cannot be
used as a guide to estimate the length of time that has elapsed since
the earth has been at a temperature capable of supporting animal and
vegetable life; for it will be shown that probably there is sufficient
radio-active matter on the earth to supply as much heat to the earth as
is lost by radiation from its surface. Taking the average conductivity
_K_ of the materials of the earth as ·004 (C.G.S. units) and the
temperature gradient _T_ near the surface as ·00037° C. per cm., the
heat _Q_ in gram-calories conducted to the surface of the earth per
second is given by
_Q_ = 4π_R²KT_,
where _R_ is the radius of the earth.
Let _X_ be the average amount of heat liberated per second per cubic
centimetre of the earth’s volume owing to the presence of radio-active
matter. If the heat _Q_ radiated from the earth is equal to the heat
supplied by the radio-active matter in the earth,
_X_ . (⁴⁄₃)π_R³_ = 4π_R²KT_,
or
3_KT_
_X_ = ------ .
_R_
Substituting the values of these constants,
_X_ = 7 × 10⁻¹⁵ gram-calories per second
= 2·2 × 10⁻⁷ gram-calories per year.
Since 1 gram of radium emits 876,000 gram-calories per year, the
presence of 2·6 × 10⁻¹³ grams of radium per unit volume, or 4·6 × 10⁻¹⁴
grams per unit mass, would compensate for the heat lost from the earth
by conduction.
Now it will be shown in the following chapter that radio-active matter
seems to be distributed fairly uniformly through the earth and
atmosphere. In addition, it has been found that all substances are
radio-active to a feeble degree, although it is not yet settled whether
this radio-activity may not be due mainly to the presence of a
radio-element as an impurity. For example, Strutt[382] observed that a
platinum plate was about ¹⁄₃₀₀₀ as active as a crystal of uranium
nitrate, or about 2 × 10⁻¹⁰ as active as radium. This corresponds to a
far greater activity than is necessary to compensate for the loss of
heat of the earth. A more accurate deduction, however, can be made from
data of the radio-activity exhibited by matter dug out of the earth.
Elster and Geitel[383] filled a dish of volume 3·3 × 10³ c.c. with clay
dug up from the garden, and placed it in a vessel of 30 litres capacity
in which was placed an electroscope to determine the conductivity of the
enclosed gas. After standing for several days, they found that the
conductivity of the air reached a constant maximum value, corresponding
to three times the normal. It will be shown later (section 284) that the
normal conductivity observed in sealed vessels corresponds to the
production of about 30 ions per c.c. per second. The number of ions
produced per second in the vessel by the radio-active earth was thus
about 2 × 10⁶. This would give a saturation current through the gas of
2·2 × 10⁻¹⁴ electromagnetic units. Now the emanation from 1 gram of
radium stored in a metal cylinder gives a saturation current of about
3·2 × 10⁻⁵ electromagnetic units. Elster and Geitel considered that
most of the conductivity observed in the gas was due to a radio-active
emanation, which gradually diffused from the clay into the air in the
vessel. The increased conductivity in the gas observed by Elster and
Geitel would thus be produced by the emanation from 7 × 10⁻¹⁰ gram of
radium. Taking the density of clay as 2, this corresponds to about 10⁻¹³
gram of radium per gram of clay. But it has been shown that if 4·6 ×
10⁻¹⁴ gram of radium were present in each gram of earth, the heat
emitted would compensate for the loss of heat of the earth by conduction
and radiation. The amount of activity observed in the earth is thus
about the right order of magnitude to account for the heat emission
required. In the above estimate, the presence of uranium and thorium
minerals in the earth has not been considered. Moreover, it is probable
that the total amount of radio-activity in the clay was considerably
greater than that calculated, for it is likely that other radio-active
matter was present which did not give off an emanation.
If the earth is supposed to be in a state of thermal equilibrium in
which the heat lost by radiation is supplied from radio-active matter,
there must be an amount of radio-active matter in the earth
corresponding to about 270 million tons of radium. If there were more
radium than this in the earth, the temperature gradient would be greater
than that observed to-day. This may appear to be a very large quantity
of radium, but recent determinations (section 281) of the amount of
radium emanation in the atmosphere strongly support the view that a
large quantity of radium must exist in the surface soil of the earth.
Eve found, on a minimum estimate, that the amount of emanation always
present in the atmosphere is equivalent to the equilibrium amount
derived from 100 tons of radium. There is every reason to believe that
the emanation found in the atmosphere is supplied both by the diffusion
of the emanation from the soil and by the action of springs. Since the
emanation loses half its activity in four days, it cannot diffuse from
any great depth. Assuming that the radium is uniformly distributed
throughout the earth, the quantity of the radium emanation produced in a
thin shell of earth about thirteen metres in depth, is sufficient to
account for the amount ordinarily observed in the atmosphere.
I think we may conclude that the present rate of loss of heat of the
earth might have continued unchanged for long periods of time in
consequence of the supply of heat from radio-active matter in the earth.
It thus seems probable that the earth may have remained for very long
intervals of time at a temperature not very different from that observed
to-day, and that, in consequence, the time during which the earth has
been at a temperature capable of supporting the presence of animal and
vegetable life may be very much longer than the estimate made by Lord
Kelvin from other data.
=272. Evolution of matter.= Although the hypothesis that all matter is
composed of some elementary unit of matter or protyle has been advanced
as a speculation at various times by many prominent physicists and
chemists, the first definite experimental evidence showing that the
chemical atom was not the smallest unit of matter was obtained in 1897
by J. J. Thomson in his classic research on the nature of the cathode
rays produced by an electric discharge in a vacuum tube. We have seen
that Sir William Crookes, who was the first to demonstrate the
remarkable properties of these rays, had suggested that they consisted
of streams of projected charged matter and represented—as he termed it—a
new or “fourth state of matter.”
J. J. Thomson showed by two distinct methods (section 50), that the
cathode rays consisted of a stream of negatively charged particles
projected with great velocity. The particles behaved as if their mass
was only about ¹⁄₁₀₀₀ of the mass of the atom of hydrogen, which is the
lightest atom known. These corpuscles, as they were termed by Thomson,
were found at a later date to be produced from a glowing carbon filament
and from a zinc plate exposed to the action of ultra-violet light. They
acted as isolated units of negative electricity, and, as we have seen,
may be identified with the electrons studied mathematically by Larmor
and Lorentz. Not only were these electrons produced by the action of
light, heat, and the electric discharge, but similar bodies were also
found to be emitted spontaneously from the radio-elements with a
velocity far greater than that observed for the electrons in a vacuum
tube.
The electrons produced in these various ways were all found to carry a
negative charge, and to be apparently identical; for the ratio _e_/_m_
of the charge of the electron to its mass was in all cases the same
within the limits of experimental error. Since electrons, produced from
different kinds of matter and under different conditions, were in all
cases identical, it seemed probable that they were a constituent part of
all matter. J. J. Thomson suggested that the atom is built up of a
number of these negatively charged electrons combined in some way with
corresponding positively charged bodies.
On this view the atoms of the chemical elements differ from one another
only in the number and arrangement of the component electrons.
The removal of an electron from the atom in the case of ionization does
not appear to affect permanently the stability of the system, for no
evidence has so far been obtained to show that the passage of an intense
electric discharge through a gas results in a permanent alteration of
the structure of the atom. On the other hand, in the case of the
radio-active bodies, a positively charged particle of mass about twice
that of the hydrogen atom escapes from the heavy radio-atom. This loss
appears to result at once in a permanent alteration of the atom, and
causes a marked change in its physical and chemical properties. In
addition there is no evidence that the process is reversible.
The expulsion of a β particle with great velocity from an atom of
radio-active matter also results in a transformation of the atom. For
example radium E emits a β particle, and, in consequence, gives rise to
a distinct substance radium F (polonium). A case of this kind, where the
expulsion of a β particle with great velocity causes a complete
rearrangement of the parts of an atom, is probably quite distinct from
the process which occurs during ionization, where a slow speed electron
escapes from the atom without apparently affecting the stability of the
atom left behind.
The only direct experimental evidence of the transformation of matter
has been derived from a study of the radio-active bodies. If the
disintegration theory, advanced to account for the phenomena of
radio-activity, is correct in the main essentials, then the
radio-elements are undergoing a spontaneous and continuous process of
transformation into other and different kinds of matter. The rate of
transformation is slow in uranium and thorium, but is fairly rapid in
radium. It has been shown that the fraction of a mass of radium which is
transformed per year is about ¹⁄₂₀₀₀ of the total amount present. In the
case of uranium and thorium probably a million years would be required
to produce a similar amount of change. Thus the process of
transformation in uranium and thorium is far too slow to be detected
within a reasonable time by the use of the balance or spectroscope, but
the radiations which accompany the transformation can easily be
detected. Although the process of change is slow it is continuous, and
in the course of ages the uranium and thorium present in the earth must
be transformed into other types of matter.
Those who have considered the possibility of atoms undergoing a process
of transformation have generally thought that the matter as a whole
would undergo a progressive change, with a gradual alteration of
physical and chemical properties of the whole mass of substance. On the
theory of disintegration this is not the case. Only a minute fraction of
the matter present breaks up in unit time, and in each of the successive
stages through which the disintegrated atoms pass, there is in most
cases a marked alteration in the chemical and physical properties of the
matter. The transformation of the radio-elements is thus a
transformation of a part _per saltum_, and not a progressive change of
the whole. At any time after the process of transformation has been in
progress there will thus remain a part of the matter which is unchanged,
and, mixed with it, the products which have resulted from the
transformation of the remainder.
The question naturally arises whether the process of degradation of
matter is confined to the radio-elements or is a universal property of
matter. It will be shown in chapter XIV that all matter, so far
examined, exhibits the property of radio-activity to a slight degree. It
is very difficult, however, to make certain that the observed
radio-activity is not due to the presence in the matter of a slight
trace of a radio-element. If ordinary matter is radio-active, it is
certain that its activity is much less than that of uranium, and
consequently that its rate of transformation must be excessively slow.
There is, however, another possibility to be considered. The changes
occurring in the radio-elements would probably never have been detected
if the change had not been accompanied by the expulsion of charged
particles with great velocity. It does not seem unlikely that an atom
may undergo disintegration without projecting a part of its system with
sufficient velocity to ionize the gas. In fact, we have seen that, even
in the radio-elements, several of the series of changes in both thorium,
radium, and actinium are unaccompanied by ionizing rays. The
experimental results given in Appendix A strongly support this point of
view. It may thus be possible that all matter is undergoing a slow
process of transformation, which has so far only been detected in the
radio-elements on account of the expulsion of charged particles with
great velocity during the change. This process of degradation of matter
continuing for ages must reduce the constituents of the earth to the
simpler and more stable forms of matter.
The idea that helium is a transformation product of radium suggests the
probability that helium is one of the more elementary substances of
which the heavier atoms are composed. Sir Norman Lockyer, in his
interesting book on “Inorganic Evolution,” has pointed out that the
spectra of helium and of hydrogen predominate in the hottest stars. In
the cooler stars the more complex types of matter appear. Sir Norman
Lockyer has based his theory of evolution of matter on evidence of a
spectroscopic examination of the stars, and considers that temperature
is the main factor in breaking up matter into its simpler forms. The
transformation of matter occurring in the radio-elements is on the other
hand spontaneous, and independent of temperature over the range
examined.
Footnote 333:
Perrin, _Revue Scientifique_, April 13, 1901.
Footnote 334:
Becquerel, _C. R._ 133, p. 979, 1901.
Footnote 335:
Rutherford and McClung, _Phil. Trans._ A, p. 25, 1901.
Footnote 336:
Rutherford, _Phil. Mag._ Jan. and Feb. 1900.
Footnote 337:
P. Curie, _C. R._ 136, p. 223, 1903.
Footnote 338:
Rutherford, _Phil. Mag._ April, 1903.
Footnote 339:
M. and Mme Curie, _C. R._ 134, p. 85, 1902.
Footnote 340:
Rutherford and Soddy, _Trans. Chem. Soc._ 81, pp. 321, 837, 1902.
_Phil. Mag._ Sept. and Nov. 1902.
Footnote 341:
Rutherford and Soddy, _Phil. Mag._ April, 1903.
Footnote 342:
Rutherford and Soddy, _Phil. Mag._ May, 1903.
Footnote 343:
Rutherford, _Phys. Zeit._ 4, p. 235, 1902. _Phil. Mag._ Feb. 1903.
Footnote 344:
Rutherford, _Phil. Mag._ May, 1903.
Footnote 345:
Curie and Laborde, _C. R._ 136, p. 673, 1903.
Footnote 346:
J. J. Thomson, _Nature_, p. 601, 1903.
Footnote 347:
Crookes, _C. R._ 128, p. 176, 1899.
Footnote 348:
F. Re, _C. R._ p. 136, p. 1393, 1903.
Footnote 349:
Richarz and Schenck, _Berl. Ber._ p. 1102, 1903. Schenck, _Berl. Ber._
p. 37, 1904.
Footnote 350:
Armstrong and Lowry, _Proc. Roy. Soc._ 1903. _Chem. News_, 88, p. 89,
1903.
Footnote 351:
Rutherford and Soddy, _Phil. Mag._ May, 1903.
Footnote 352:
Boltwood, _Nature_, May 25, p. 80, 1904. _Phil. Mag._ April, 1905.
Footnote 353:
McCoy, _Ber. d. D. Chem. Ges._ No. 11, p. 2641, 1904.
Footnote 354:
Strutt, _Nature_, March 17 and July 7, 1904. _Proc. Roy. Soc._ March
2, 1905.
Footnote 355:
Strutt, _Proc. Roy. Soc._ March 2, 1905.
Footnote 356:
Soddy, _Nature_, May 12, 1904; Jan. 19, 1905.
Footnote 357:
Whetham, _Nature_, May 5, 1904; Jan. 26, 1905.
Footnote 358:
Danne, _C. R._ Jan. 23, 1905.
Footnote 359:
J. J. Thomson, _Nature_, April 30, p. 601, 1903.
Footnote 360:
Voller, _Phys. Zeit._ 5, No. 24, p. 781, 1904.
Footnote 361:
Ramsay and Cooke, _Nature_, Aug. 11, 1904.
Footnote 362:
Eve, _Nature_, March 16, 1905.
Footnote 363:
J. J. Thomson, International Electrical Congress, St Louis, Sept.
1904.
Footnote 364:
Rutherford and Soddy, _Phil. Mag._ p. 582, 1902; pp. 453 and 579,
1903.
Footnote 365:
Ramsay and Soddy, _Nature_, July 16, p. 246, 1903. _Proc. Roy. Soc._
72, p. 204, 1903; 73, p. 346, 1904.
Footnote 366:
Curie and Dewar, _C. R._ 138, p. 190, 1904. _Chem. News_, 89, p. 85,
1904.
Footnote 367:
Himstedt and Meyer, _Ann. d. Phys._ 15, p. 184, 1904.
Footnote 368:
Strutt, _Proc. Roy. Soc._ March 2, 1905.
Footnote 369:
Boltwood, _Phil. Mag._ April, 1905.
Footnote 370:
Moss, _Trans. Roy. Soc. Dublin_, 1904.
Footnote 371:
Travers, _Nature_, p. 248, Jan. 12, 1905.
Footnote 372:
Jaquerod, _C. R._ p. 789, 1904.
Footnote 373:
Ramsay and Travers, _Zeitsch. Physik. Chem._ 25, p. 568, 1898.
Footnote 374:
Ramsay, _Nature_, April 7, 1904.
Footnote 375:
Lodge, _Nature_, June 11, p. 129, 1903.
Footnote 376:
Larmor, _Aether and Matter_, p. 233.
Footnote 377:
J. J. Thomson, _Phil. Mag._ p. 681, Dec. 1903.
Footnote 378:
Lord Kelvin, _Phil. Mag._ Oct. 1904.
Footnote 379:
Thomson, _Phil. Mag._ March, 1904.
Footnote 380:
Rutherford and Soddy, _Phil. Mag._ May, 1903.
Footnote 381:
See Strutt and Joly, _Nature_, Oct. 15, 1903.
Footnote 382:
Strutt, _Phil. Mag._ June, 1903.
Footnote 383:
Elster and Geitel, _Phys. Zeit._ 4, No. 19, p. 522, 1903. _Chem.
News_, July 17, p. 30, 1903.