Chapter VII: Radio-Active Emanations (3)

The experiments were repeated with another capillary tube and the volume of gas observed at normal pressure was 0·0254 c. mm. The gas obtained was found to obey Boyle’s law within the limit of experimental error over a considerable range of pressure. But, unlike in the first experiment, the gas did not contract but expanded rapidly during the first few hours, and then more slowly, finally reaching a volume after 23 days of 0·262 c. mm. or about 10 times the initial volume. The measurements were complicated by the appearance of bubbles of gas in the top of the mercury column. The differences observed in these two experiments are difficult to account for. We shall see, later, that the emanation always produces helium, and, in the first experiment, the decrease of the volume to zero indicates that the helium was buried or absorbed in the walls of the tube. In the second case, probably owing to some difference in the glass of the capillary tube, the helium may have been released. This suggestion is confirmed by the observation that the volume of gas, after the experiment ended, gave a brilliant spectrum of helium.

We shall see later that there is considerable evidence that the α particles expelled from radio-active substances consist of helium atoms. Since the particles are projected with great velocity, they will first be buried in the walls of the tube, and then may gradually diffuse out into the gas again under conditions probably depending on the kind of glass employed. Since α particles are projected from the emanation and also from two of the rapidly changing products which arise from it, the volume of helium should, on this view, be three times the initial volume of the emanation. If the helium produced escaped from the walls of the tube into the gas, the apparent volume of the gas in the capillary should increase to three times the initial volume in a month’s interval, for during that time the emanation itself has been transformed into a solid type of matter deposited on the walls of the tube.

Ramsay and Soddy concluded from their experiments that the maximum volume of emanation to be obtained from 1 gram of radium was about 1 cubic millimetre at standard pressure and temperature, and that the emanation was produced from 1 gram of radium at the rate of 3 × 10⁻⁶ c. mm. per second. This amount is in very good agreement with the calculated value, and is a strong indication of the general correctness of the theory on which the calculations are based.

=173. Spectrum of the emanation.= After the separation of the emanation and the determination of its volume, Ramsay and Soddy made numerous attempts to obtain its spectrum. In some of the earlier experiments several bright lines were seen for a short time, but these lines were soon masked by the appearance of the hydrogen lines. In later experiments Ramsay and Collie[267] succeeded in obtaining a spectrum of the emanation, which persisted for a short time, during which a rapid determination of the wave-lengths was made. They state that the spectrum was very brilliant, consisting of very bright lines, the spaces between being perfectly dark. The spectrum bore a striking resemblance in general character to the spectrum of the gases of the argon family.

The spectrum soon faded, and the spectrum of hydrogen began to appear. The following table shows the wave-length of the lines observed in the spectrum. The degree of coincidence of the lines of known wave-lengths shows that the error is probably less than five Ångström units.

Wave-length Remarks

6567 Hydrogen C; true wave-length, 6563; observed each
time.

6307 Observed only at first; evanescent.

5975 „ „ „

5955 „ „ „

5805 Observed each time; persistent.

5790 Mercury; true wave-length, 5790.

5768 „ „ 5769.

5725 Observed only at first; evanescent.

5595 Observed each time; persistent and strong.

5465 Mercury; true wave-length, 5461.

5105 Not observed at first; appeared after some
seconds; persisted and was visible
during the second examination.

4985 Observed each time; persistent and strong.

4865 Hydrogen F; true wave-length, 4861.

4690 Observed only at first.

4650 Not observed when the emanation was examined
again.

4630 „ „ „

4360 Mercury: true wave-length, 4359.

The experiments were repeated with a new supply of emanation, and some of the stronger lines were observed again, while some new lines made their appearance. Ramsay and Collie suggest that the strong line 5595 may be identical with a line which was observed by Pickering[268] in the spectrum of lightning, and was not identified with the spectrum of any known gas.

Until large quantities of radium are available for the experimenter it would appear difficult to make sure how many of these lines must be ascribed to the spectrum of the emanation or to measure the wave-lengths with accuracy.

The results are of great interest, as showing that the emanation has a definite and new spectrum of the same general character as the argon group of gases to which, as we have seen, it is chemically allied.

Summary of Results.

=174.= The investigations into the nature of the radio-active emanations have thus led to the following conclusions:—The radio-elements thorium, radium and actinium continuously produce from themselves radio-active emanations at a rate which is constant under all conditions. In some cases, the emanations continuously diffuse from the radio-active compounds into the surrounding gas; in other cases, the emanations are unable to escape from the material in which they are produced, but are occluded, and can only be released by solution or by the action of heat.

The emanations possess all the properties of radio-active gases. They diffuse through gases, liquids, and porous substances, and can be occluded in some solids. Under varying conditions of pressure, volume, and temperature, the emanations distribute themselves in the same way and according to the same laws as does a gas.

The emanations possess the important property of condensation under the influence of extreme cold, and by that means can be separated from the gases with which they are mixed. The radiation from the emanation is material in nature, and consists of a stream of positively charged particles projected with great velocity.

The emanations possess the property of chemical inertness, and in this respect resemble the gases of the argon family. The emanations are produced in minute amount; but a sufficient quantity of the radium emanation has been obtained to determine its volume and its spectrum. With regard to their rates of diffusion, the emanations of both thorium and radium behave like gases of high molecular weight.

These emanations have been detected and their properties investigated by the property they possess of emitting radiations of a special character. These radiations consist entirely of α rays, _i.e._ particles, projected with great velocity, which carry a positive charge and have a mass about twice that of the hydrogen atom. The emanations do not possess the property of permanently radiating, but the intensity of the radiations diminishes according to an exponential law with the time, falling to half value, from actinium in 4 seconds, from thorium in one minute, and from radium in about four days. The law of decay of activity does not seem to be influenced by any physical or chemical agency.

The emanation particles gradually break up, each particle as it breaks up expelling a charged body. The emanation after it has radiated ceases to exist as such, but is transformed into a new kind of matter, which is deposited on the surface of bodies and gives rise to the phenomena of excited activity. This last property, and the connection of the emanation with it, are discussed in detail in the next chapter.

Footnote 231:

Owens, _Phil. Mag._ p. 360, Oct. 1899.

Footnote 232:

Rutherford, _Phil. Mag._ p. 1, Jan. 1900.

Footnote 233:

Rossignol and Gimingham, _Phil. Mag._ July, 1904.

Footnote 234:

Bronson, _Amer. Journ. Science_, Feb. 1905.

Footnote 235:

_Phil. Mag._ April, 1904.

Footnote 236:

Dorn, _Abh. der. Naturforsch. Ges. für Halle-a-S._, 1900.

Footnote 237:

P. Curie, _C. R._ 135, p. 857, 1902.

Footnote 238:

Rutherford and Soddy, _Phil. Mag._ April, 1903.

Footnote 239:

P. Curie, _C. R._ 136, p. 223, 1903.

Footnote 240:

Debierne, _C. R._ 136, p. 146, 1903.

Footnote 241:

Giesel, _Ber. D. deutsch. Chem. Ges._ p. 3608, 1902.

Footnote 242:

Curie and Debierne, _C. R._ 132, pp. 548 and 768, 1901.

Footnote 243:

Curie and Debierne, _C. R._ 133, p. 931, 1901.

Footnote 244:

Rutherford and Soddy, _Trans. Chem. Soc._ p. 321, 1902. _Phil. Mag._
Sept. 1902.

Footnote 245:

Rutherford, _Phys. Zeit._ 2, p. 429, 1901.

Footnote 246:

Rutherford and Soddy, _Phil. Mag._ Nov. 1902.

Footnote 247:

Rutherford and Soddy, _Phil. Mag._ April, 1903.

Footnote 248:

Rutherford and Soddy, _Phil. Mag._ Nov. 1902.

Footnote 249:

Rutherford and Soddy, _Phil. Mag._ April, 1903.

Footnote 250:

Curie and Debierne, _C. R._ 133, p. 931, 1901.

Footnote 251:

Rutherford and Soddy, _Phil. Mag._ Nov. 1902.

Footnote 252:

Ramsay and Soddy, _Proc. Roy. Soc._ 72, p. 204, 1903.

Footnote 253:

Rutherford and Miss Brooks, _Trans. Roy. Soc. Canada 1901_, _Chem.
News 1902_.

Footnote 254:

Loschmidt, _Sitzungsber. d. Wien. Akad._ 61, II. p. 367, 1871.

Footnote 255:

See Stefan, _Sitzungsber. d. Wien. Akad._ 63, II. p. 82, 1871.

Footnote 256:

P. Curie and Danne, _C. R._ 136, p. 1314, 1903.

Footnote 257:

Bumstead and Wheeler, _Amer. Jour. Science_, Feb. 1904.

Footnote 258:

Makower, _Phil. Mag._ Jan. 1905.

Footnote 259:

Wallstabe, _Phys. Zeit._ 4, p. 721, 1903.

Footnote 260:

Stefan, _Wien. Ber._ 2, p. 371, 1878.

Footnote 261:

Rutherford and Soddy, _Phil. Mag._ Nov. 1902.

Footnote 262:

_Phil. Mag._ May, 1903.

Footnote 263:

P. Curie, Société de Physique, 1903.

Footnote 264:

Rutherford and Soddy, _Phil. Mag._ May, 1903.

Footnote 265:

_Nature_, Aug. 20, 1903.

Footnote 266:

_Proc. Roy. Soc._ 73, No. 494, p. 346, 1904.

Footnote 267:

_Proc. Roy. Soc._ 73, No. 495, p. 470, 1904.

Footnote 268:

Pickering, _Astrophys. Journ._ Vol. 14, p. 368, 1901.