We have also seen that the emanation from radium diffuses into the air according to the law of the diffusion of gases, and with a coefficient of diffusion comparable with that of carbon dioxide. Finally, the emanations from radium and thorium condense at low temperatures like liquefiable gases.
At the same time it should be remembered that no one has yet observed any pressure due to the emanation, nor has any one shown by weighing that a material gas is present. All our knowledge of the properties of the emanation results from measurements of radioactivity. More than that, no one has yet shown with certitude that there is a characteristic spectrum produced by the emanation.
The emanation should also not be considered as an ordinary material gas, for it disappears spontaneously from a sealed tube containing it, and the rapidity of its disappearance is absolutely independent of the conditions of the experiment, especially of the temperature.
It is very curious that the numerous attempts made under the most varied conditions to obtain chemical reactions with the emanation have been fruitless. To explain this fact Rutherford thinks that the emanations are gases of the argon family.
The following facts are difficult to explain. The emanation from radium condenses at -150°, and according to Rutherford, a current of air at -153° may be passed over it continuously without removing it. The amount of the emanation must be very small, and if it had the slightest tension at -153°, it would quickly evaporate in a current of air. Further, the temperature of condensation by cooling should be a function of the amount of the emanation in a given volume of air, which has not yet been proved.
Debierne and I have found that the emanation passes with extreme ease through the tiniest holes and fissures in solids, while under the same conditions the ordinary gases could circulate only with the greatest slowness.
Rutherford supposes that radium destroys itself spontaneously, and that the emanation is one of the products of the breaking-down. Debierne and I observed that a solid salt of radium quite rapidly excites the walls of the reservoir filled with air, which contains it, by the emanation which it emits. On the contrary, if a quite perfect vacuum be made in the reservoir the excitation takes place only with extreme slowness; but it rapidly reappears when a gas has been admitted. However, the emanation spreads much more rapidly in a gas at very low pressure than at the atmospheric pressure. It seems from this that the emanation experiences some particular difficulty in escaping from radium which is in a vacuum.
Disengagement of Gas by the Salts of Radium. Production of Helium.—Giesel noticed that solutions of radium bromide continually give off gases. These gases are principally hydrogen and oxygen, in the same relative proportion as in water, and might, therefore, come from the decomposition of the water of the solution. But Ramsay and Soddy also showed that there always is a small quantity of helium that they detected by its spectrum in a Geissler tube. The helium lines were also accompanied by three unknown lines.
A solid salt of radium also constantly gives off gases capable of producing a pressure in a closed tube. To this liberation of gas can be attributed two accidents during my experiments. A sealed bulb of thin glass, almost filled with well-dried bromide of radium, exploded and became slightly warm at the same time. An explosion was also caused by dry radium chloride when heated quite rapidly in a vacuum to 300°. In this case the explosion seemed to be caused by fragments of the solid salt filled with occluded gas.
At the moment we dissolve in water a solid salt of radium that has been prepared a long time, there is an abundant evolution of gas.