i8o CLASSICS OF MODERN SCIENCE 

 as the number of molecules of the first substance. Accordingly, on 

 our hypothesis when a gas combines with two or more times its 

 volume of another gas, the resulting compound, if gaseous, must 

 have a volume equal to that of the first of these gases. Now, in 

 general, this is not actually the case. For instance, the volume of 

 water in the gaseous state is, as M. Gay-Lussac has shown, twice as 

 great as the volume of oxygen which enters into it, or, what comes 

 to the same thing, equal to that of the hydrogen instead of being equal 

 to that of the oxygen. But a means of explaining facts of this type 

 in conformity with our hypothesis presents itself naturally enough : 

 we suppose, namely, that the constituent molecules of any simple 

 gas whatever (i. e., the molecules which are at such a distance from 

 each other that they cannot exercise their mutual action) are not 

 formed of a solitary elementary molecule, but are made up of a cer- 

 tain number of these molecules united by attraction to- form a single 

 one; and further, that when molecules of another substance unite 

 with the former to form a compound molecule, the integral molecule 

 which should result splits up into two or more parts (or integral 

 molecules) composed of half, quarter, &c., the number of elementary 

 molecules going to form the constituent molecule of the first sub- 

 stance, combined with half, quarter, &c., the number of constituent 

 molecules of the second substance that ought to enter into combina- 

 tion with one constituent molecule of the first substance (or, what 

 comes to the same thing, combined with a number equal to this last 

 of half -molecules, quarter-molecules, &c., of the second substance) ; 

 so that the number of integral molecules of the compound becomes 

 double, quadruple, &c., what it would have been if there had been 

 no splitting-up, and exactly what is necessary to satisfy the volume 

 of the resulting gas. 



On reviewing the various compound gases most generally knov/n, 

 I only find examples of duplication of the volume relatively to the vol- 

 ume of that one of the constituents 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 is twice that of the oxygen. 

 Finally, nitrous gas, which contains equal volumes of nitrogen and 



