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considered, we have, if we take neon as 139



VIII



THE OTHER INACTIVE GASES



263









Helium.


Nitrogen.
297 x 1


Oxygen.
270 x 1


Fluorine.
192 x 1


139 x J
Neon.
139 x 1


Phosphorus.
297 x4


Sulphur.
270x4


Chlorine.
192x4


Argon.
139x4


Arsenic.
1


Selenium.
1


Bromine.
192 x6


Krypton.
139 x 6


Antimony.
?


Tellurium.
?


Iodine.
192 x 10


Xenon.
139 x 10



These numbers are values of the expression
(p 1) x 10 6 , and are proportional to the retarda-
tion of light in passing through equal numbers
of molecules of the gases, or of the elements in
the gaseous state. Their significance has not yet
met with any explanation, but it is evident that
the exceedingly simple relation must be connected
with some fundamental facts relating to the con-
stitution of matter.

The same periodicity is manifest with other
properties, such as the melting and boiling-points
of argon, krypton, and xenon, the critical tem-
peratures, and, indeed, the whole curve of vapour-
pressures. Moreover, the properties of these
elements are functions, not merely of their atomic
weights considered in reference to each other, but



-

264 THE GASES OF THE ATMOSPHERE CHAP.

also in reference to those of other elements : thus,
for example, while the atomic volume of sulphur
(atomic weight, 32) is 21*6, that of chlorine (35*5)
is 23'5, that of argon (39'9) is 32*9, and that of
potassium (39*1) 45*4.

But it is the electric behaviour of these new
elements which has most interest. For while
fluorine, chlorine, bromine, and iodine are the most
electro-negative of the elements, being separated
from their compounds with metals at the positive
pole, and while elements of the sodium group,
namely, lithium, sodium, potassium, rubidium,
and caesium, are the most electro-positive of the
elements, separating at the negative pole when a
solution of one of their compounds, or the fused
compound, provided it is a conductor of electricity,
is electrolysed, these elements occurred in con-
tiguous columns in the periodic table of the
elements. Now it is difficult to see how elements
of such opposite properties should be next each
other, without some transition. " Natura nihil
fit per saltum " ; and it would be reasonable to
expect a bridge to unite columns containing
elements of such opposite properties. This bridge
has now been discovered : it consists of the neutral



vui THE OTHER INACTIVE GASES 265

elements of the argon group, which have no electric
polarity, seeing that they form no compounds. It
is owing to this neutrality and to their low boiling-
points that they occur in the atmosphere. The
boiling-point of any substance appears to be influ-
enced greatly by its molecular weight, as well as
by the nature of the elements forming the com-
pound ; and these gases, being molecularly simple
(for their molecules are identical with their atoms),
have especially low boiling-points, and therefore
occur only as permanent gases.

It is curious that although the presence of
helium is revealed in the sun and in many of the
fixed stars by its spectrum, that of argon has not
been detected. This leads to the suspicion that a
hypothesis put forward by Dr. Johnstone Stoney
contains a considerable element of truth. It is
that gases are continually leaving our atmosphere,
owing to the intrinsic rate of motion of their mole-
cules. A molecule of hydrogen, for example, when
it arrives at the confines of our atmosphere, may
escape, provided its rate of motion is sufficiently
rapid. And it may be proved that some molecules
of hydrogen possess sufficient velocity to carry
them beyond the sphere of the earth's attraction ;



266 THE GASES OF THE ATMOSPHERE CHAP.

it would follow that, given sufficient time, all mole-
cules of hydrogen would ultimately fly off and
would find a home when they reached a body of
sufficient mass, and therefore of sufficient attractive
force to retain them permanently. Such a body is
the sun ; and it has been abundantly proved that
free hydrogen exists in quantity in the solar
atmosphere. But M. Gautier and Lord Kayleigh
have shown that our terrestrial atmosphere con-
tains a detectible quantity of free hydrogen ; and
Sir William Ramsay and Dr. Travers have proved
it to contain helium. Why do these gases remain
in our atmosphere ? Why, in the course of ages, do
they not leave it, each molecule pursuing its way
as an independent wanderer, until it comes under
the sway of the sun, or of some planet of sufficient
mass to retain it in its atmosphere ?

The answer to this question must be that
hydrogen and helium are continually being evolved
from the earth in such quantity as to replenish the
drainage of these gases into space. The existence
of helium in the gases from mineral waters leads
to the very probable guess that that gas must be
escaping in appreciable quantity from the soil ; and
it is well known that hydrogen is produced by



viri THE OTHER INACTIVE GASES 267

imperfect combustion, and thus finds its way into
our atmosphere.

The absence of the spectrum of argon from the
sun's atmosphere is more puzzling. The explana-
tion may perhaps lie in the fact that the spectrum
of argon is easily masked by that of other gases.
It is impossible to see argon lines in a mixture
containing a small amount of nitrogen ; and the
spectrum is much enfeebled, too, if oxygen be
present in the vacuum-tube. If this is not the
explanation, it must be concluded that the relative
quantity of argon in the sun's chromosphere is
small compared with that in the atmosphere of the
earth ; or possibly that the compounds of argon
are stable at the enormously high temperature of
the sun, a suggestion which has something in its
favour ; for compounds which are formed with
absorption of heat acquire greater stability the
higher the temperature ; and it is not inconceivable
that although argon, at atmospheric temperature,
and under atmospheric conditions, refuses to com-
bine, it may yet form compounds under the much
greater extremes of electric disturbances and high
temperature which obtain in the sun and in the
fixed stars. It is only recently that the spectrum



268 THE GASES OF THE ATMOSPHERE CHAP.

of oxygen has been recognised in the sun, and a
possible reason of its feebleness may be the stability
of some of its compounds endothermic under
normal conditions.

Shortly after the wave-lengths of the lines in
tne spectrum of krypton were published, Sir William
Huggins, in a private letter,, suggested to Sir Wil-
liam Kamsay that its brilliant green line appeared
to be identical with that seen in the spectrum of
the aurora borealis. The same remark was made
somewhat later by Professor Schuster, in a letter
to Nature.

The aurora borealis or Northern Lights gene-
rally appears in the north, on frosty evenings, as a
luminous arch, from which streamers descend, and
emit light, sometimes white, sometimes green, and
sometimes crimson. The height of this arch appears
to be from 50 to 125 miles. The spectrum contains
numerous lines, all of which have been shown by
Mr. Baly to be identical with strong lines in the
spectrum of krypton, but the strongest is one of
wave-length 5570 Angstrom units.

Now this krypton line persists at great rare-
factions. Even when the amount of krypton is
reduced to one twenty-three-millionth part of its



viii THE OTHER INACTIVE GASES 269

normal pressure, the line still is visible. It can
be calculated that the pressure of the atmosphere
would be equal to that amount at a height of 80
miles, a number which falls within the limits given
above.

Sir William Earn say has succeeded in producing
an artificial aurora by causing a ring-shaped dis-
charge to take place through krypton in the
interior of a flask, and by a powerful electro-
magnet, suitably placed, the " streamers" can also
be reproduced. Such an aurora shows all the
peculiarities of the natural aurora, including the
spectrum characteristic of krypton.

The progress of events has resulted in the dis-
covery of new atmospheric gases, the peculiarity
of which is their short life ; and the next chapter
will be devoted to a description of their sources
and properties.



!



CHAPTER IX



THE RADIOACTIVE GASES : THE " EMANATIONS "



THE year 1896 was remarkable for the discovery
by M. Henri Becquerel that the metal uranium
and its salts were capable of impressing a photo-
graphic plate, even after they had been kept for
years in the dark; they appeared to be able to
emit a constant and unceasing flow of something
analogous to light. Moreover, the rays emitted
were found to discharge an electrified body, so that
no charged object could retain its charge in their
immediate neighbourhood. But the effect of such
" uranium rays " was feeble. Madame Curie, two
years later, announced that she had succeeded in
extracting from pitchblende, the natural ore of
uranium, a metal resembling bismuth, to which
she gave the name " polonium " a word derived
from Poland, of which she is a native. This sup-



270



CHAP, ix THE RADIOACTIVE GASES 271

posed metal possessed these properties of uranium,
but in a much higher degree. Shortly afterwards,
she announced the discovery of another element
still more active, and happily named it " radium."
Compounds of the element thorium were also
found to exhibit similar properties; and later,
another substance was separated from pitchblende
by M. Debierne and by Professor Giesel, named
by the former "actinium," and by the latter
" emanium." The property of emitting " rays,"
which were at first imagined to resemble those of
light, has been termed "radioactivity."

Perhaps the simplest way to test for radio-
activity is to place the substance under examination
on the top of an ordinary photographic plate,
wrapped up in black paper, so that no light can
reach it. But the photographic method is not well
fitted for quantitative experiments. A much more
satisfactory instrument is the gold-leaf electroscope.
It consists of a metal chamber with two windows
of glass or mica opposite each other, through which
the gold-leaf can be observed. A tin oil-can forms
an efficient chamber. It is closed by an india-
rubber cork, perforated with two holes. Through
one hole passes a thin brass rod, to the lower end



272 THE GASES OF THE ATMOSPHERE CHAP.

of which a short rod of fused silica is attached. A
slip of brass, about -% inch thick, \ inch wide, and
2J inches long, is cemented on to the lower end of
the silica rod ; and attached by a dash of gum to
the upper end of this brass slip is a strip of gold
leaf of the same length and width, which hangs
down parallel to the brass slip, so long as it is not
electrified, but when charged, the gold leaf stands
out more or less from the brass slip like an A
with one leg vertical (the strip), and the thin leg
representing the gold leaf. To impart a charge to
the gold leaf and strip, a stiff brass wire passes
through the second hole in the india-rubber cork ;
this wire is bent at the lower end, so that on
twisting it round, the lower end makes contact
with the brass slip. By rubbing a piece of ebonite
or sealing-wax, it is charged, and on touching the
wire with it, the wire being in contact with the
brass slip, the latter is charged, and the gold leaf
diverges. The wire is then twisted, so as to break
contact with the slip ; and it is then advisable to
connect the charging wire to earth, by means of
a piece of thin wire attached to a gas-pipe. The
bottom of the oil can should be removable ; and it
is well to pierce the india-rubber cork with a third



ix THE RADIOACTIVE GASES 273

hole through which a glass tube passes, in order
that it may be possible to introduce a gas into the
metal chamber. If a solid is to be tested for radio-
activity, the electroscope is charged, and the solid
is laid on the bottom, which is then replaced. Eays
from the radioactive substance have an effect on
the air contained in the can, termed " ionisation " ;
ionised air has the property of discharging an
electrified body ; and the amount of ionisation, and
therefore the rate of discharge, is proportional to
the intensity of the radiation of the radioactive
substance. If a radioactive gas is to be tested, a
measured quantity is blown through the glass tube
into the can ; it will discharge the electroscope
more or less quickly according to the extent of its
radioactivity. By observing the rate of fall of the
charged gold leaf through a telescope fitted with
a scale in its eye-piece, comparative experiments
may be made, and the relative radioactivity of two
substances compared.

More accurate measurements may be made with
an electrometer ; but enough has been said to give
a fair idea of a practicable method of testing for
and estimating radioactivity.

Compounds of radium, thorium, and uranium



274 THE GASES OF THE ATMOSPHERE CHAP.

differ from those of uranium and polonium in that
they continuously evolve gases ; but these gases
are unlike others with which we are acquainted,
for they decompose or disintegrate in a short time.
Only one of the products of such decomposition has
been identified with any known chemical element ;
it is helium, which is produced from the gas evolved
from compounds of radium. To these gases the name
" emanation " has been given by Professor Ruther-
ford, the discoverer of the first of these to be ob-
served namely, the emanation from thorium. The
thorium emanation, like other gases, mixes with air,
and air, thus mixed, acquires and retains the property
of discharging an electroscope, so long as the emana-
tion remains undecomposed. In conjuction with
Mr. Frederick Soddy, Rutherford showed that the
emanation can be condensed by passing it through
a tube cooled below - 154 C. by means of liquid
air ; this, as the reader has observed, is now a
familiar method of separating two gases from each
other.

Monsieur and Madame Curie observed that radium
compounds, too, had the power of imparting radio-
activity, lasting for a considerable time, to air with
which they were in contact ; but they did not divine



ix THE RADIOACTIVE GASES 27$

the true cause of the radioactivity namely, the
evolution of a gas. The radium -gas was also
investigated by Kutherford and Soddy, and found
to be condensable, like the thorium gas.

In 1900, Professor Geitel and Mr. C. T. R.
Wilson independently discovered that a positively
or negatively charged body, placed in a closed
vessel, gradually lost its charge. And Elster and
Geitel in 1901 tried to extract the radioactive
substance from the air, believing that the loss of
charge in the closed vessel was due to some radio-
active constituent of the atmosphere. Their method
of extraction depended on an observation made by
Rutherford, that the gas from thorium was attracted
by a negatively charged object, and could be made
to deposit its decomposition-product on it ; and as
this decomposition-product is also radioactive, its
presence could be detected, and its comparative
quantity measured. The same method would
attract the radium emanation, and lead to its
detection.

This method of detecting radioactive substances
by means of their discharging power is incomparably
more delicate than the most delicate chemical or
spectroscopic test ; the amount which is at the



276 THE GASES OF THE ATMOSPHERE CHAP.

disposal of the experimenter is extremely small.
Moreover, the products of their decomposition
are also quite invisible. Their presence or absence
can be detected only by their power of ionising
air, and thus affecting an electroscope : and they
are differentiated from each other by the time
during which they retain that power. For example,
the radium emanation, mixed with air, is measured ;
a known fraction of the whole that is, a certain
number of cubic centimetres of the radioactive gas
is blown into an electroscope, and the rate at which
the leaf falls is measured. That quantity contains
a certain amount of emanation which we shall call
x ; its absolute amount is unknown. The stock of
air is now kept for 92 '6 hours, and a fresh portion,
the same in volume as the former, is blown into
the electroscope. It is again discharged, but the
time required is twice as long as the first, for one
half of the emanation has been changed into pro-
ducts which are non-volatile but also radioactive.
If these deposit on the sides of the vessel con-
taining the electroscope, the vessel would become
radioactive ; hence it is necessary, after the first
measurement, to remove the bottom of the vessel, and
take care to expel the residual emanation completely



ix THE RADIOACTIVE GASES 277

by a current of air. After a second period of 92' 6
hours, a third equal quantity of the emanation,
which has now been kept for 185*2 hours, is
admitted : the time required for discharge is now
four times as long as the original time that is,
only one quarter of the original emanation has
survived. An example of actual measurements
is given below.



Age of
Emanation
in hours.


Conductivity
of Air in
Electroscope.


Age of
Emanation
in hours.


Conductivity
of Air in
Electroscope.





. 345


241 .


. 57-5


123-5 .


. 143


316 .


. 31-4


168 .


96-2


363 .


. 19-5


196


75-2


484-5 .


. 7-7



The conductivity is inversely proportional to the
time of discharge of the electroscope ; it is evident
that it falls off as the emanation grows older
and until it reaches nearly a zero value.

The law of decrease is a well known one ; it
may be likened to the inverse of the law of com-
pound interest. If a sum of money is lent, it is
customary to pay interest on it at stated intervals ;
for example, a yearly interval is usual. Thus, at
four per cent per annum, a sum of 100 yields, at
the end of a year, 4 interest. Supposing it to be



278 THE GASES OF THE ATMOSPHERE CHAP,

agreed that the intrest is to be payable at the rate
of four per cent per half-year, then it is clear that
100 in six months will have increased to 102.
During the second six months, the interest at 4 per
cent accrues not on 100, but on 102 ; it is there-
fore 2, i.e. the interest on 100 for six months,
plus about 9|d., the interest on 2 for six months
at 4 per cent. If it be agreed to pay at the rate of
4 per cent per quarter, then the interest works out
as follows:

Interest on ,100 for 3 months at 4 p.c. = 100

\ 101 00 = 1 2 T V

101 2- T V =10 4i

101 4J =10



By this method, therefore, the interest at the end
of the year will be increased by about Is. l^d. If
the period of payment were monthly, instead of
quarterly, it is clear that the sum gained would be
still larger ; if daily, hourly, once every minute,
once every second, the increase would be pro-
gressively greater. Stated generally, the rate of
increase of the principal at any moment depends
on the amount of the principal at that moment.
Now the decay of the emanation is an inverse



ix THE RADIOACTIVE GASES 279

case of this. Let us take a supposititious instance.
Imagine, for simplicity's sake, that in each day one
tenth of the total quantity of emanation present
decomposes. Then we should have

T - Amount of Emanation Amount Decomposed

present. per day.

0-1 I'OOO O'l x 1-000 = 0-100

1 - 2 (1-000 - O'lOO) = 0-900 0-1 x 0'900 = 0-090

2-3 (0-900 - 0-090) = O'SIO O'l x 0'890 = 0-081

3-4 (0-810 - 0-081) = 0-729 O'l x 0-729 = 0-073

4-5 (0-729 - 0-073) = 0'656 O'l x 0'656 = 0'066

etc. etc. etc.

At each moment the amount decomposing, how-
ever, is proportional to the quantity present. Now
the mathematical expression for this is

It -xt
T = e

A o

where I is the amount present at the beginning of
the change, I t , the amount present in time, t, e a
number equal to 2718 . . ., and X a constant.
The value of \ may be defined as the reciprocal of
the average life of a particle of the emanation.

In the case of the radium emanation, A, = f^^j^i
it means that that proportion of the total amount
of emanation present decomposes in a second. The
average life of a particle of this emanation is there-



280 THE GASES OF THE ATMOSPHERE CHAP.

fore 463,000 seconds, or 5 days, 9 hours. The
thorium emanation has a much shorter life : it is
87 seconds, and X has the value -g^. Still shorter is
the life of the emanation from actinium : it is only
5 '8 seconds; nearly ^th of the whole emanation
decomposes each second.

The gases from a solution of a radium salt (the
bromide is commonly used) consist for the most
part of a mixture of oxygen and hydrogen ; about
10 cubic centimetres per gram of radium per day
are evolved. There is always a small excess of
hydrogen, which amounts to about 6 per cent of
the total volume of the mixed gases. The gases
are evidently derived from the decomposition of
the water of the solution ; but it is not easy to
account for the excess of hydrogen : it may be due
to the formation of bromate of radium, Rd(Br0 3 ) 2 ,
although this has not been satisfactorily ascer-
tained. Mixed with these gases is the emanation,
in extremely minute amount.

Rutherford and Soddy investigated to some
extent the action of chemical agents on the thorium
and radium emanations. In each case they resemble
the inert gases of the atmosphere. Copper oxide
at a red heat, red-hot zinc dust, and red-hot



ix THE RADIOACTIVE GASES 281

platinum black in presence of oxygen are without
effect on them ; their discharging power, property
of condensation, etc., are unaffected. And Kamsay
and Soddy confirmed this evidence of their indif-
ference towards reagents : neither sparking with
oxygen in presence of caustic potash, nor passage
over red-hot magnesium-lime mixture in any way
altered the radium emanation. It must, therefore,
be concluded that they resemble most the gases of
the argon group in this respect.

Now it is remarkable that those elements which
display radioactivity have all a very high atomic
weight. Thus radium belongs to the barium series,
with a probable atomic weight of 226 ; thorium is
allied to silicon, but has the high atomic weight
232 ; and uranium is the element with the highest
atomic weight known 240.

It has been shown, however, by Dr. Hahn,
working under Kamsay's direction, that the im-
pure thoria from thorianite contains a substance
for which the name " radiothorium " has been
proposed, of intense radioactivity. Indeed, it
appears to bear to thoria the same sort of relation-
ship as radium to uranium. It was separated,
along with radium, from a sample of a mineral from



282 THE GASES OF THE ATMOSPHERE CHAP.

Ceylon, named thorianite. This substance was left
behind along with barium and radium sulphates,
after the mineral had been fused with hydrogen
sodium sulphate. Its bromide is more soluble than
that of radium ; moreover, unlike radium, it is
precipitable by ammonia. The quantity obtained
is too small to have made it possible to determine
its atomic weight ; its activity, however, is at least
half-a-million times that of the crude thoria from
which it was separated. The emanation which it
evolves is identical in every respect with that evolved
from salts thorium ; and the natural conclusion is
that it is the substance to which thorium compounds
owe their radioactivity. It is still doubtful whether
thorium has a radioactivity of its own, like uranium ;
the assertion has been made by two different observers
that thorium oxide from certain minerals is non-
radioactive. Like thorium salts, too, salts of this
substance give on precipitation with ammonia a
filtrate containing thorium X, a body which is at
first strongly radioactive, but which soon loses its
radioactivity ; while the precipitate from which the
thorium X has been separated is less radioactive,
and regains its activity at a rate precisely the
same as that at which the thorium X loses activity.



ix THE RADIOACTIVE GASES 283

We do not know the atomic weights of the
gases evolved from radiothorium or from actinium.
That of "niton," the gas evolved from radium, is,
however, known, for its density has been deter-
mined by Kamsay and Whytlaw-Gray. Experi-
ments made with the view of determining this
quantity by comparing their rates of diffusion with
those of gases of known density can hardly be
pronounced satisfactory. Perhaps the best of such
experiments is the one made by Debierne with
"niton," the name now adopted instead of the
cumbrous "radium emanation." He caused the
gas to issue through a minute hole in a diaphragm
of platinum, and found the molecular weight of


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