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the niton to be nearly 220.

The density, and consequently the molecular
weight, of niton was determined in 1910 by Kamsay
and Whytlaw-Gray, by means of a " microbalance,"
an instrument constructed for the purpose of
weighing exceedingly minute quantities. A good
ordinary balance, constructed with a brass or
aluminium beam resting on an agate knife-edge
bearing on an agate plate, turns with the tenth
part of a milligram, when the pans are loaded
with 20 or 30 grams. An assay-balance, made on



284 THE GASES OF THE ATMOSPHERE CHAP.

essentially the same principle, but with a much
lighter beam, may turn with ^th or even T <hjth
of a milligram. But such a degree of sensitiveness
is by no means sufficient for the determination of
the weight of the niton produced from an obtain-
able weight of radium. The volume of niton in
equilibrium with 1 gram of metallic radium is only
0'6 cubic millimetre; and Kamsay and Whytlaw-
Gray had about the sixth part of this at their
disposal, 0*1 cubic millimetre, a quantity barely
visible with the unaided eye in a capillary tube.
Its weight was found to be about 2 thousandths
of a milligram ; and in order to reach a reason-
ably accurate result, the balance required should
be able to show the 200th part of that weight, or
the 1/100, 000th of a milligram. The sensitiveness
attained was indeed five times as great ; the balance
" turned" with the 500,000th part of a milligram.
The beam was constructed of thin silica rods, fused
together with an oxy-coal-gas blowpipe ; its shape
was like that of an ordinary balance. It rested
on a knife-edge of polished silica bearing on a
polished silica plate. From one end of the beam
hung a counterpoise of silica by a silica fibre ;
from the other a hook, to which the apparatus



ix THE RADIOACTIVE GASES 285

could be suspended. The advantage of silica as
a material for the balance is its hardly being
affected by temperature ; it barely expands when
heated; besides, it is very strong, and very thin
fibres of silica can support comparatively heavy
weights. To the centre of the balance was fused a
small silvered mirror, from which a beam of light
could be reflected on to a vertical scale ; this served
as a pointer, and had the great advantage that the
pointer is weightless, for a beam of light has no
weight. The balance was enclosed in an air-tight
brass case, resting on a brass plate ; a vacuum could
be made in the case, and air could be admitted,
so as to have any required air-pressure ; the actual
pressure was read by means of a mercury gauge.

Hanging suspended by a silica fibre from the
end of the beam opposite to that carrying the
counterpoise was a small silica bulb ; the capacity
of the bulb used was 22*2 cubic millimetres. It
contained air, weighing 0*027 milligram. Now
this air would be counterpoised, as it were, by the
air in the balance-case, if the latter contained air
at atmospheric pressure ; but if all air were with-
drawn from the balance-case, it would be equivalent
to putting a weight of 0*027 milligram on the end



286 THE GASES OF THE ATMOSPHERE CHAP.

of the beam. For intermediate pressures the
apparent weight of the air in the bulb is pro-
portional ; thus at a pressure of half an atmosphere,
or 380 millimetres of mercury, the air in the little
bulb would act as a weight of '027/2, or 0'0136
milligram. It was possible to read a difference
of pressure on the gauge of 0*1 millimetre, or
1/7 6 00th part of the atmospheric pressure ; and
this corresponded to putting on or taking off a
weight of 0-027/7600 milligram, or 0'00000355
milligram ; that is a little more than 3 millionths
of a milligram.

The niton was generated (along with oxygen
and hydrogen) from about half a gram of bromide
of radium in 8 or 9 days ; after explosion, the
residue consisted of a little hydrogen containing
the niton. It was forced up by mercury into a
very narrow capillary tube, the tip of which was
then cooled with liquid air, so as to freeze the
niton. The volume was then greatly increased, so
as to diminish to insignificance the amount of
hydrogen in the capillary tube ; and this was then
sealed off, " full " of niton.

The capillary tube was then placed in a little
glass "bucket," suspended from the balance, and



(ix ^/ THE RADIOACTIVE GASES 287

counterpoised by a suitable piece of silica rod,
hung on the opposite arm of the balance. Silica
is slightly volatile ; so after suitable pieces had
been drawn off from this piece of rod, originally
too heavy, exact counterpoise was obtained by
directing the tip of the blowpipe flame on what
remained, until the counterpoise was exact. The
air-pressure was then adjusted so that complete
equilibrium ensued.

Next the "bucket" was removed with forceps,
and the pointed end of the capillary tube was
broken by pressure in the " bucket " ; this made
certain that no little pieces of glass could scatter,
for the drawn-out part of the capillary tube fitted
the "bucket" pretty accurately. The "bucket"
and capillary were then replaced on the balance,
and the air-pressure in the case adjusted until
equilibrium again set in. The weight of the niton
was then ascertained in terms of the buoyancy
of the air in the little bulb, and that was translated
into actual weight. The volume of the niton was
calculated from measurements previously carried
out with radium, the amount produced in a given
time being known.

The mean of five such weighings gave the



288 THE GASES OF THE ATMOSPHERE CHAP.

molecular weight of niton as 223 ; and as the
molecular weight of a monatomic gas is the same
as its atomic weight (see pp. 200-216), it follows
that the atomic weight of niton is also 223.

But Ramsay and Why tlaw- Gray carried the
matter further. Radium, when it changes into
niton, parts with an atom of helium ; niton, in
its turn, yields Radium A, and another atom of
helium is shot off ; Radium A changes into Radium
B, but no helium is here lost; Radium B gives
Radium C, and again an atom of helium separates.
All these changes are fairly rapid ; in three months,
it may be said, appreciably all the niton has
changed into Radium D, which is a pretty per-
manent body, keeping its identity for many years.
Is it possible to find the loss of weight caused by
the loss of helium ?

A sealed capillary tube was left in the " bucket"
of the balance for four months, and was then
counterpoised. By that time all the niton had
changed into solid Radium D and helium. The
tip of the capillary was broken as usual, and a
vacuum was made in the balance-case so as to
pump all the helium out of the capillary ; the
weight was again taken ; the loss was 27 million ths



ix THE RADIOACTIVE GASES 289

of a milligram. But the weight of the helium
from the amount of niton in the tube should have
been 38 millionths.

It has been mentioned that the molecules (or
atoms) of niton " explode " when they change into
Eadium A ; and the other products, B and C, shoot
out each an atom of helium during their change.
Was it not possible that some of the helium had been
shot into the walls of the glass capillary ? That
was easily tested. The capillary tube was placed
inside a little tube of quartz, and all air was
removed by pumping. A little oxygen was then
admitted. Then heat was applied, so as to fuse
the glass capillary, and all gas was pumped off.
The oxygen was condensed out by cooling a bulb
containing cocoa-nut charcoal in connection with
the tube containing the gas, and the small residue
of helium was measured, and from the volume
the weight was calculated ; it came to 8 millionths
of a milligram. Adding 8 to the 27 millionths
previously weighed, the total weight of the helium
is 35 millionths of a milligram, instead of the
calculated 38. And making the little measuring-
tube into a Pliicker tube by wrapping tin -foil
round its ends, and passing a discharge, the



2QO THE GASES OF THE ATMOSPHERE CHAP.



spectrum of pure helium was seen. Hence the
idea that niton, in changing into radium D, gives
off three atoms of helium was proved to be
correct.

Dr. Whytlaw-Gray and the writer have also
liquefied and solidified niton, observing it in a
capillary tube through a microscope. The liquid,
which can be produced by compression at the
ordinary temperature, shines with a bright purple
light ; the solid, formed by cooling the liquid
with liquid air, looks like a miniature arc-light a
very brilliant luminous point. The melting-point
of the solid and the boiling-point of the liquid
were determined.

It would be desirable to carry out a similar
series of experiments with the emanations from
thorium and actinium, but unfortunately it is
impossible ; these substances are too transient,
and cannot be kept long enough to handle. We
have therefore no direct knowledge of their atomic
weights, and can only speculate.

An experiment made with actinium emanation
appears to point to its being a light gas. If a
piece of paper containing an actinium salt (of
course in a very impure condition, for salts of



ix THE RADIOACTIVE GASES 291



actinium have never been obtained free from
impurities) be held about the middle of a card-
board screen, covered over with phosphorescent
zinc sulphide, at about half an inch from the screen,
it can be seen that the gas rises. Suppose the
screen to be represented by the line, and the paper
by the dot, then in the position . there is hardly
any luminosity produced on the screen. In the
position ~7~ the screen becomes intensely luminous.
In the positions / and X the evidence is again
that the emanation rises, care of course being taken
to avoid convection currents of air. The actinium
emanation is particularly well adapted for such
experiments, for its effect is very transitory ; it can
be blown away, and it takes an appreciable time to
reappear. The phenomenon is a very striking one,
and conveys the conviction that an invisible
substance is streaming on to the screen to excite
luminosity where it touches the phosphorescent
covering. Now, if the gas from actinium is lighter
than air, it must have a density of considerably
less than 14 '5 (hydrogen being taken as unity).
This, however, is very improbable ; the upward
streaming is doubtless due to the heat of the
actinium preparation expanding the air.



292 THE GASES OF THE ATMOSPHERE CHAP.

It has been already mentioned that radium
emanation undergoes decomposition, and that one
of its products is helium. Professor Kutherford
and Mr. Soddy, after finding that thorium emana-
tion was an indifferent gas, remarked : " The
speculation naturally arises whether the presence
of helium in minerals and its invariable association
with uranium and thorium may not be connected
with their radioactivity." The state in which
helium is retained in minerals, too, is analogous
to the state in which radium emanation is retained
in salts of radium. It cannot be pronounced to be
combined ; it is almost certainly molecularly en-
tangled in the interior of the solid, and escapes
only when heat is applied. This state has been
imitated by decomposing a compound of nitrogen,
mixed with a solid, when the nitrogen is similarly
retained.

The discovery that radium emanation, during
its change, is converted partially into helium was
the result of an unsuccessful attempt to obtain
the spectrum of the emanation. It was at first
imagined that if the emanation were mixed with
a gas of simple spectrum, such as helium, its lines
might be visible. But it soon became evident



ix THE RADIOACTIVE GASES 293

that the amount of emanation present was so small
that specially minute apparatus would have to be
constructed in order to deal with it. The Pliicker
tube, instead of being of dimensions measurable
in inches, was constructed in dimensions measurable
in millimetres (1 inch = 25 millimetres). And the
volume tube, in which the amount of emanation
obtainable from a given weight of radium in a
given time was measured, was of the finest capillary
tubing. It has been mentioned that the gases
evolved from a solution of radium bromide consist
of a mixture of hydrogen and oxygen in nearly the
proportion of two to one, with a small excess of
hydrogen, and that the emanation is mixed with
these gases. The gases evolved in eight days from
60 milligrams of radium bromide in aqueous solution
were exploded by a spark in a small explosion
burette ; the residue of hydrogen was left in con-
tact with moist caustic potash for some time in
order that any carbon dioxide which might have
arisen from dust on the surface of the glass being
burnt, should be absorbed. The residual hydrogen,
containing the emanation, passed through a tube
containing phosphorus pentoxide so as to remove
water -vapour; and it was driven upwards, by



294 THE GASES OF THE ATMOSPHERE CHAP.

means of mercury, until it entered a small bulb,
to the top part of which a capillary tube of known
bore was sealed. The bulb was then cooled with
liquid air, so as to condense the emanation, and
the hydrogen was entirely removed by pumping.
The jacket of liquid air was then removed, and the
emanation evaporated ; by raising the mercury
column, it was made to enter the capillary tube,
where it was measured.

It was a brightly luminous gas ; its volume was
nearly the fortieth of a cubic millimetre that is,
the forty -thousandth of a cubic centimetre. It
was found to follow Boyle's law the volume
decreased in proportion as the pressure was in-
creased. From day to day the volume decreased,
until after four weeks less than one two-
thousandth of a cubic millimetre was left. This
minute bubble, however, was as brightly luminous
as at first, although, of course, there was much
less of it.

The mercury was drawn down the capillary
tube into the bulb, and was there frozen. On
heating the glass so as to expel helium, and passing
a discharge, the helium spectrum was visible. It
appears then that the emanation contracts to



ix THE RADIOACTIVE GASES 295

practically nothing in about four weeks ; that
helium is formed from it, which penetrates the
walls of the glass tube, probably because the mole-
cules are shot off with enormous velocity ; and
that, on heating, some at least of the helium is
expelled, so that it can be recognised by its
spectrum.

In a second experiment, in which a capillary
tube of a different kind of glass was used, there
was no contraction, but, on the contrary, an ex-
pansion. The initial volume of emanation was
practically identical with that which had previously
been found ; but it expanded to about ten times
its original volume in three weeks. The gas was
then removed by pumping, and it showed a
brilliant helium spectrum ; some gas had also
been absorbed.

It is easy to calculate from these data the
amount of emanation produced from one gram of
radium. It appears to be about one cubic centi-
metre from one gram in a year. But as this goes
on, the radium will continually decrease in weight,
and hence the actual amount of emanation evolved
will continually diminish, again according to the
inverse law of compound interest. It can be cal-



296 THE GASES OF THE ATMOSPHERE CHAP.

culated that the average life of an atom of radium
is about 1730 years, supposing that the only pro-
duct of its initial decomposition (not of subsequent
changes, in which the emanation is concerned) is
emanation.

Now if radium is. changing at this rate, its pro-
duction must keep pace with its waste ; else it
would all have disappeared during the enormous
period of time of existence of the minerals in
which it is found. The natural supposition is that
uranium, the main constituent of the pitchblende
in which radium invariably occurs, may be changing
continuously into radium. This conjecture is
strengthened by the fact that the amount of radium
in such minerals always bears a constant ratio to
that of uranium. But very careful experiments
by Mr. Soddy, in which he freed a solution of a
uranium salt completely from radium by repeated
precipitation of the radium with sulphuric acid in
presence of a barium salt, showed that the rate of
formation is by no means so rapid. That radium
is formed appeared to be proved ; but not at the
expected rate.

The spectrum of the radium emanation has also
been observed. The process of obtaining the pure



ix THE RADIOACTIVE GASES 297

gas was practically the same as the one already
described; but a minute Pliicker's tube was sub-
stituted for the capillary measuring tube. The
character of the spectrum is analogous to that of
the inert gases ; it is characterised by a number
of clearly-cut lines, chiefly in the green region.
Their position has been accurately mapped by
Watson and others. These lines have been observed
to be present in the spectra of many of the fixed
stars.

The disruption of the radium molecule is
accompanied by a relatively enormous heat evolu-
tion. Eutherford has found that of this heat,
75 per cent is derived from the emanation and its
subsequent products of change. The Curies found
that 1 gram of radium evolves no less than 100
calories per hour ; hence 1 '3 cubic millimetres, the
amount of emanation yielded per hour by one
gram of radium, must be responsible for 75 calories.
Comparing this with the heat evolved by a violent
chemical change of the ordinary character, the
difference is enormous. One cubic centimetre of
emanation, were it possible to obtain it, would
evolve about seven and a half million calories
during its complete change ; while a cubic centi-



298 THE GASES OF THE ATMOSPHERE CHAP.

metre of mixed oxygen and hydrogen gases evolves
on explosion only 2'05 calories, or 3*4 million times
less than the heat of disruption of an equal volume
of emanation that is, on the ordinary assumption
of an equal number of molecules. The process of
change of an atom, therefore, while the same in
kind as an ordinary chemical reaction, differs
entirely in the magnitude of the result : the
amount of energy parted with during the dis-
ruption of an atom is hardly commensurable with
that due to its combination with another atom to
form a compound.

That a charged electroscope always leaks on
standing, and is slowly discharged, has been a
subject of annoyance to physicists ever since its
invention. It was for long supposed to be due to
damp. As a matter of fact, if the insulating rod
from which the gold-leaf is suspended be of glass,
the discharge of the electroscope is closely connected
with the hygrometric state of the atmosphere ;
for glass is attacked by water, especially if, as is
always the case in natural air, carbon dioxide is
present. The solution of carbonic acid, no doubt,
decomposes the glass, forming sodium carbonate ;
and in moist weather a conducting film is formed



ix THE RADIOACTIVE GASES 299

on the glass consisting of a solution of that salt.
Hence leakage takes place along the supposed
insulator. But if material be employed which is
not acted on by water, such as sulphur, ebonite,
amber, or fused quartz, the electroscope still slowly
discharges ; and it has often been shown that
the moisture of the air has no connection with
the observed leakage. The earth appears to be
negatively electrified : for if a wire, in contact
with the earth, extends upwards into the air, a
negative charge continuously leaks away. And
the leakage, if the wire be insulated from the
earth, is less in damp than in dry weather ; indeed
it is least during a fog. The ions of atmospheric
gases are trapped by the small water-particles of a
fog and removed ; and as the leakage depends
on the pressure of such ions, anything which
diminishes the number of ions in the air produces
a corresponding diminution of leakage of electricity.
Now, negative ions induce the formation of fog. If
air containing moisture be cooled, the moisture will
condense. In order that it may condense, unless the
degree of cooling be very great, nuclei are necessary.
These nuclei may be minute solid particles, such
as those of smoke ; or they may be ions, either



300 THE GASES OF THE ATMOSPHERE CHAP.

positive or negative. The negative ions attract
the fog particles, and induce liquefaction at a
higher temperature than do the positive ions.
Hence the negative ions are removed as the fog
precipitates, leaving the positive ions still free in
the atmosphere. And thus these positive ions
discharge a negatively electrified body.

Owing to the rarity of radium and thorium, it
was long ere it was suspected that the ionisation
of the atmosphere was due to their presence in the
soil. But as soon as it was discovered that radio-
active gases are products of their disintegration, it
became evident that it is owing to the presence
of such gases that the atmosphere acquires its
discharging power. It was observed by Elster
and Geitel, to whom most of our knowledge of
atmospheric electricity is due, that the air in
underground cellars has a much greater discharging
power than that of the free atmosphere ; and this
suggested a source for the ionisation of the air
namely, the presence of a greater amount of radio-
active gas.

Now Eutherford showed that when the thorium-
emanation decomposes, its products are attracted
by a negatively electrified body, such as a platinum



ix THE RADIOACTIVE GASES 301

wire connected with the negative pole of a battery
of several hundred volts. Elster and Geitel, there-
fore, tested for such products by connecting an
insulated wire, suspended in the air, with such a
source of negative potential for several hours ; and
it was easy to prove, by coiling the wire and
transferring it to an electroscope, that the " induced
activity " due to the presence of the decomposition
product of radium emanation was present on it.
That this is really a material coating on the wire
is shown by the fact that it can be removed by
polishing the wire with sand-paper, or by treating
it with an acid, so as to dissolve off the superficial
layer. The coating is specially rich if the nega-
tively charged wire is exposed to air from subter-
raneous sources. The activity has usually been
traced to radium emanation, though proof has been
obtained that the decomposition of thorium emana-
tion is also responsible for the radioactivity. This
can be done by measuring the period of decay of
the " induced activity," which differs in the two
cases.

Closely connected with these phenomena is the
radioactivity of mineral waters. The waters of
Bath and of Harrogate, as well as those of some



302 THE GASES OF THE ATMOSPHERE CHAP.

German mineral springs, have been shown to
contain radium emanation in solution ; and Strut t
has found that the deposit on the sides of the Bath
springs contains a minute trace of radium. J. J.
Thomson, too, has proved that water from deep
wells near Cambridge is radioactive. Indeed,
radium appears to be a widely- spread element,
although it occurs in almost infinitesimal quantity.
Such waters, no doubt, give off their emanation to
the air ; and as the life of the radium emanation
may be taken as about a month, much survives in
the air and, by ionising its gaseous constituents,
confers on them the power of discharging an
electroscope. The emanation from thorium, or
rather from radiothorium, has a much shorter
period of life ; it may be reckoned as at the most
ten minutes ; hence its radioactive power is soon
exhausted, and it can be detected only near the
soil. It is a remarkable fact that thorium emana-
tion is produced from some soils in quantity much
greater than can be accounted for by the thorium
which they contain, and it is to be presumed
that this is due to the presence of radiothorium
in quantity far too small for detection by any
analytical process.



ix THE RADIOACTIVE GASES 303

One other fact remains to be noted in relation
to the gases of the argon group : we can tell the
origin of helium and neon in the atmosphere.
Much, if not all, of the helium is the product of
the change of radium, thorium, and actinium ;
whence comes the neon ?

It was noticed in certain experiments carried
out by Cameron and Eamsay, on the action of
niton on water, that besides decomposing water
into its constituent elements, oxygen and hydrogen,
and besides setting free helium, which was easily
recognised by its spectrum, the spectrum of neon
was observable, so bright that it could not have
been due to mere traces. The theory was put
forward that both helium and oxygen being
"nascent," that is, in process of being formed, it
was not inconceivable that combination might
occur; the atomic weight of oxygen, 16, plus that


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Online LibraryWilliam RamsayThe gases of the atmosphere, the history of their discovery → online text (page 15 of 16)