660 SCIENCE PROGRESS 



degree of freedom experimentally established. In fact, the 

 meaning of the 5/3rds ratio of the specific heats involves a 

 knowledge first of the monatomicity of the gas molecule before 

 it can be used to discuss the larger questions of the absence 

 of rotational energy and the thermal isolation of the atomic 

 structure from its environment. 



Let us now turn to the other side of the shield, and consider 

 the progress of knowledge in other directions since Prof. 

 Armstrong made his suggestion in 1895. The electron, as the 

 atom of negative electricity first definitely proved to be capable 

 of existence apart from matter by Sir J. J. Thomson, has to 

 be reckoned with, together with the evidence that the electron 

 is a constituent of all atoms, and is responsible for their chemical 

 affinities and the vast majority of their physical properties. Do 

 the argon gases contain these electrons? It might be thought 

 that the absence of chemical affinities and the zero number of 

 this family in the periodic grouping indicated complete absence 

 of electrons, or, at least, of electrons capable of being detached 

 from the atom. Whereas the fact that the a-particle has two 

 atomic charges of positive electricity is proof that the helium 

 atom has two electrons capable of being detached, at least 

 during radioactive changes. It is, in this respect, analogous 

 to an element of the alkaline-earth family, and the question 

 arises, therefore, why compounds of the type ACL and HeCl 2 

 are not possible. 



Nor is it only in the more drastic changes of radioactivity 

 that this valency is manifested. The positive ray in helium, 

 for example, shows that the atom carries two positive charges. 

 Helium, when it is specially pure, exhibits what may be de- 

 scribed as a peculiar electrical inertness, in that at low pressure 

 it conducts the discharge with great difficulty. In the purest 

 helium all the phenomena of a high vacuum — production of 

 cathode-rays and of X-rays — are exhibited under the discharge 

 at a pressure, 0*5 mm. of mercury, when other gases conduct 

 with the maximum facility. At first it appeared possible that 

 absolutely pure helium might not conduct the discharge at all, 

 but a careful investigation showed that this inertness is rela- 

 tive only. Five to ten molecules of helium are electrically 

 equivalent to one of a common gas like hydrogen and nitrogen, 

 so that if the discharge in pure helium is compared at any 

 pressure with that in a common gas, not at the same pressure 



