THE ARGON FAMILY OF GASES 661 



but at a pressure five to ten times less, the same discharge 

 phenomena are observed in the two cases. This explains the 

 original observation of Sir William Ramsay and Prof. Collie 

 that helium at atmospheric pressure conducts the discharge with 

 far greater facility than other gases. (Soddy and Mackenzie, 

 Proc. Roy. Soc, 1908, 80 A, 92.) The fact, though it has never 

 been adequately accounted for, shows clearly that the electrical 

 inertness of helium is relative only, and therefore one may 

 conclude that its chemical inertness is also only relative. The 

 molecule possesses detachable electrons, but no chemical agency 

 has yet succeeded in detaching them. 



The recognition of detachable electrons as a normal con- 

 stituent of the atom alters the significance of the latter term. 

 The term atom, as it is used by chemists, now signifies a complex 

 of one material particle — the positive ion of the element — 

 together with a certain number of electrons. The single un- 

 combined material particle is the positive ion, not the atom. 

 It would be idle to pretend, in spite of the now generally 

 accepted dictum, that " the forces of chemical affinity and elec- 

 tricity are one and the same," that we have yet a complete 

 explanation of the nature of chemical affinity in terms of the 

 electron. But the numbers of the families in the Periodic Table 

 from O to VII, representing the maximum positive valency of 

 the elements, do probably represent also the maximum number 

 of electrons in the ring systems detachable in chemical changes. 

 That they do not always represent the whole number of detach- 

 able electrons in any change is shown by the case of the inert 

 gases. They are of relative rather than absolute significance, 

 and represent how one atom will behave with regard to another. 

 If an electrically neutral atom, that is, the complex of the positive 

 ion with its electrons, is the most stable compound of that ion 

 which can exist, the atom will appear to be chemically inert or 

 devoid of affinity, as in the case of the gases of the zero group. 

 In proportion as this electrically neutral complex is unstable, 

 so will the chemical activity of the element increase. Just as 

 chemists suppose that the peculiar inertness of nitrogen is best 

 accounted for on the view that the elementary nitrogen molecule, 

 the complex, N 2 , is the most stable and readily formed compound 

 of all compounds containing nitrogen, and that the single 

 nitrogen atoms have intense affinity for one another, so it 

 seems reasonable to regard the helium molecule, the compound 



