xv THE MOLECULAR THEORY 339 



molecules of a compound ; a gaseous element could 

 therefore never give rise to more than its own volume of 

 a gaseous compound. But Gay-Lussac had shown that 

 " the volume of water in the gaseous state is ... twice 

 as great as the volume of oxygen which enters into it," 

 i.e. i volume of oxygen produces 2 volumes of steam, 

 or (on the theory of equal spacing) i molecule of oxygen 

 produces 2 molecules of steam. 



" But a means of explaining facts of this type in con- 

 formity with our hypothesis presents itself naturally enough : 

 we suppose namely that the constituent molecules of any 

 simple gas . . . are not formed of a solitary elementary [atom], 

 but are made up of a certain number of these [atoms] 

 united by attraction to form a single [molecule] " (A.C.R. 

 IV. 31). 



When the element enters into combination, this molecule 

 may be split into two or more parts, giving rise to two or more 

 molecules of a compound, which would thus occupy a volume 

 two or more times as great as that of the element. 



Avogadro pointed out that the molecule of an element does 

 not usually split into more than two parts, although 

 rightly enough he asserts the possibility of further sub- 

 division. 



" On reviewing the various compound gases most 

 generally known, I only find examples of duplication of the 

 volume relatively to the volume of that one of the con- 

 stituents which combines with one or more volumes of the 

 other. We have already seen this for water. In the same 

 way, we know that the volume of ammonia gas is twice 

 that of the nitrogen which enters into it. M. Gay-Lussac 

 has also shown that the volume of nitrous oxide is equal to 

 that of the nitrogen which forms part of it, and consequently 

 twice that of the oxygen. Finally, nitrous gas, which contains 

 equal volumes of nitrogen and oxygen, has a volume equal 

 to the sum of the two constituent gases, that is to say, 



z 2 



