INERT CONSTITUENTS OF THE ATMOSPHERE. 591 



the molecular weight of argon), through 1°, allowing it to expand 

 while it is being heated, is 5 calories. Deducting the 2 calories required 

 for external work, 3 calories remain as the heat required to raise the 

 temperature of the gas. For oxygen, on the other hand, the heat re- 

 quired to raise the temperature of 32 grams, its molecular weight, 

 through 1° is 7 calories; and deducting 2 as before, the remainder is 

 5, the specific heat of oxygen. Hence for argon and for oxygen, we 

 have the properties : 



Heat Requibed. 

 No external work. External work. 

 Argon 3 : 5 :: 1 : 1% 



Oxygen 5 : 7 :: 1 : 1% 



The argument stands thus: The heat required to raise the tem- 

 perature of argon without expansion can be accounted for entirely on 

 the supposition that it is wholly used in causing the molecules to move 

 through space; on the other hand, more heat requires to be communi- 

 cated to oxygen than to argon in the proportion of 3 to 5. With 

 oxygen and similar gases, this extra heat must be doing something; 

 it is supposed to produce motion of the atoms within the molecule. 

 There is no such motion within the argon molecule; hence it is con- 

 cluded that the molecule consists of a single atom; and in. that case, 

 the molecular weight is the same as the atomic weight. The molecule 

 of oxygen may be considered as possessing a structure like that of a 

 dumb-bell; the atoms forming the knobs at each end of the bar. On 

 throwing a dumb-bell through space, it will not merely change its 

 position as a whole; but it will rotate. But a molecule of argon or 

 helium is imagined to have the simpler form of a sphere or ball ; when 

 it is thrown practically no energy is used in causing it to rotate, but 

 it is all expended in making it pass through space. 



I must apologize for introducing such abstruse conceptions into a 

 popular exposition; but they are necessary to the argument; and I 

 am afraid that no simpler means can be found of reaching the conclu- 

 sion that the molecules of argon and of helium are identical with their 

 atoms. 



As 4 is the molecular weight of helium, and as 40 is that of argon, 

 these numbers also stand for their atomic weights. Let us next see 

 how these figures fit into the periodic table. 



In 1897, as president of the Chemical Section of the British Associa- 

 tion, I chose the title ^An Undiscovered Gas' for the address to the Sec- 

 tion. The arguments in favor of the existence of such a gas were briefly 

 these: The differences between the atomic weights of consecutive 

 elements in the columns of the periodic table are approximately 16 to 

 20; thus 16.5 is the difference between the atomic weights of fluorine 

 and chlorine; 16, between those of oxygen and sulphur, and so on. 



