Miss H. T. Brooks. M.A.
(Read May 23, 1901.)
In a recent number of the Comptes Rendus, an account was given by
Curie of the evidence of the existence of a new gas from radium,
which possesses remarkable physical properties. A specimen of very
radioactive radium was placed in a glass vessel connected with a
mercury pump. On exhausting to a low vacuum and allowing the apparatus
to stand, the pressure steadily increased. When the very small volume
of gas thus collected flowed along the glass tubes, it rendered them
phosphorescent, and if left in for some time, rapidly blackened them.
The gas itself was powerfully radioactive, _i.e._ it continuously
gave out a type of Röntgen rays, which made gases partial conductors
of electricity and rapidly acted on a photographic plate. This gas
preserves its radioactive power for several weeks.
For some time past, one of the authors had been independently
investigating one of the most remarkable properties of radioactive
substances, namely, the power of continuously emitting radioactive
particles of some kind. The term "emanation" was applied to the
substance thus emitted, as there was no evidence at the time whether
the material emission was a vapour of the substance, a radioactive gas,
or particles of matter each containing a large number of molecules.
The "emanation" from thorium compounds was shown to retain its
radioactivity for several minutes and possessed the remarkable property
of causing every substance in the neighbourhood of the thorium to
become itself radioactive for several days. The specimens of impure
radium then in the possession of the author, did not possess the power
of emitting such an emanation; but Dorn, using a later and more active
preparation of radium, showed that it possessed the same emanating
power as thorium. One of the most interesting properties of excited
radioactivity is that it can be concentrated in an electric field on
the kathode, so that a very fine wire of any metal can be made to act
like a powerfully radioactive substance for several days.
A short time ago, one of the authors published an account of the effect
of temperature on the emanating power of radioactive substances, in the
Physikalische Zeitschrift. In the paper it was shown that the emanating
power of thorium increased with rise of temperature to about a red
heat, but on heating to a white heat the emanating power was destroyed
and could not be recovered. An examination of a specimen of radium
obtained from De Haen, Hanover, showed that the effect of temperature
on its emanating power was very large. When the substance was heated
below a red heat, its emanating power increased over 10,000 times, but
was to a large extent destroyed by heating to a higher temperature.
The emanation, obtained by passing a slow current of air over heated
radium, was found to preserve its radioactive powers for weeks, when
kept in a closed metal vessel. The radioactivity slowly decayed, but
was still quite appreciable after a month's interval.
The question now arose, if any physical experiments could be devised
to settle the problem as to whether the emanation was in reality a
radioactive gas, driven off from the substance, or a vapour of the
substance, or a material emission of particles much larger than
molecules. Experiments on thorium showed that no appreciable volume
of gas could be collected by leaving thorium oxide in a vacuum tube
connected with a mercury pump. No new lines were observed in the
spectrum of the gas. The amount of the emanation thus given off was
thus too small to detect by its volume in this way, but the electrical
conductivity produced by the emanation in the gas, with which it is
mixed, is often very large and can be used as a measure of the amount
of emanation present. The emanation gives out a type of radiation which
ionizes the surrounding gas. When a strong electric field is applied,
the current through the gas reaches a maximum value, and is then a
measure of the total number of ions produced per second, multiplied by
the charge on the ion.
By determining the rate of diffusion of the emanation into air or
other gases, using the electrical method, it is possible to obtain
an approximate estimate of its molecular weight. The coefficient of
inter-diffusion of most of the simple gases have long been known, and
the results show the coefficient of diffusion of one gas into another
is inversely proportional to the square root of the product of the
molecular weights. If therefore the coefficients of diffusion of the
emanation into air is found to have a value lying between that of two
gases \(A\) and \(B\), we can conclude that the molecular weight of the
emanation lies between the molecular weights of \(A\) and \(B\).
The apparatus employed was similar to that used by Loschmidt[1] in his
experiments on the coefficient of inter-diffusion of gases in the year
1871.
Fig. (1) shows the general arrangement. A long brass cylinder \(A B\)
6 cms . in diameter, 73 cms. long, was divided into two equal parts
by a movable metal slide \(S\). The ends of the cylinder were closed
with ebonite stoppers. Two insulated brass rods \(a\) and \(b\), each
half the length of the tube, passed through the ebonite stoppers and
were supported centrally in the tube. The cylinder was insulated and
connected to one pole of a battery of 300 volts, the other pole of
which was to earth. The central rods could be connected to a sensitive
quadrant electrometer.
The cylinder was covered with a thick layer of felt, and placed inside
a metal box filled with cotton wool, in order to keep temperature
conditions as steady as possible.
[Illustration: Fig. I.]
In order to carry a sufficient quantity of emanation into the half
cylinder \(A\), it was necessary to slightly heat the radium. The slide
\(S\) was closed and the side tubes opened. A slow current of dry
air from a gas bag, passed through a platinum tube, in which a small
quantity of a radium compound was placed. The emanation was carried
with the air into the cylinder \(A\). When a sufficient quantity had
been introduced, as tested by the electrometer, the current of air was
stopped. The side tubes were closed by fine capillary tubes. These
prevented any appreciable loss of gas due to diffusion, but served
to keep pressure of gas inside \(A\) at pressure of outside air. The
three entrance tubes into the cylinder, shown in the figure, were for
the purpose of initially mixing the emanation and gas as uniformly as
possible.
After standing for several hours to make temperature conditions steady,
the slide was opened, and the emanation began to diffuse into the tube
\(B\).
The current through the tubes \(A\) and \(B\) was measured by an
electrometer, with suitable capacity in parallel, at regular intervals.
Initially there is no current in \(B\), but after the opening of the
slide, the amount in \(A\) decreased and the amount in \(B\) steadily
increased. After several hours the amount in each half is nearly
the same, showing that the emanation is nearly uniformly diffused
throughout the cylinder.
It can be readily shown that if
\[
\begin{aligned}
K &= \text{coefficient of diffusion of the emanation into air}.\\
f &= \text{duration of diffusion experiments in secs}.\\
a &= \text{total length of cylinder}.\\
S &= \text{amount of emanation in tube}\, A\, \text{at end of diffusion}.\\
S_{2}& = \text{amount of emanation in tube}\, B\, \text{at end of diffusion, then}
\end{aligned}
\]
\[
\frac{S_{1}-S_{2}}{S_{1}+S_{2}}=\frac{8}{\pi^{2}}\left\{e^{\frac{-\pi^{2} K t}{a^{2}}}+\frac{1}{9} e^{\frac{-\pi^{2} K t}{a^{2}}}+\text { etc. }\right\}
\]