178 CLASSICS OF MODERN SCIENCE 

 simple proportion by volume, and that when the result of the union 

 is a gas, its volume also is very simply related to those of its com- 

 ponents. But the quantitative proportions of substances in com- 

 pounds seem only to depend on the relative number of molecules which 

 combine, and on the number of composite molecules which result. 

 It must then be admitted that very simple relations also exist be- 

 tween the volumes of gaseous substances and the numbers of simple 

 or compound molecules which form them. The first hypothesis to 

 present itself in this connection, and apparently even the only ad- 

 missible one, is the supposition that the number of integral molecules 

 in any gases is always the same for equal volumes, or always pro- 

 portional to the volumes. Indeed, if we were to suppose that the 

 number of molecules contained in a given volume were different for 

 different gases, it would scarcely be possible to conceive that the law 

 regulating the distance of molecules could give in all cases relations 

 so simple as those which the facts just detailed compel us to acknowl- 

 edge between the volume and the number of molecules. On the other 

 hand, it is very well conceivable that the molecules of gases being at 

 such a distance that their mutual attraction cannot be exercised, their 

 varying attraction for caloric may be limited to condensing a greater 

 or smaller quantity around them, without the atmosphere formed 

 by this fluid having any greater extent in the one case than in the 

 other, and, consequently, without the distance between the molecules 

 varying; or, in other words, without the number of molecules con- 

 tained in a given volume being different. Dalton, it is true, has pro- 

 posed a hypothesis directly opposed to this, namely, that the quantity 

 of caloric is always the same for the molecules of all bodies whatso- 

 ever in the gaseous state, and that the greater or less attraction for 

 caloric only results in producing a greater or less condensation of 

 this quantity around the molecules, and thus varying the distance be- 

 tween the molecules themselves. But in our present ignorance of 

 the manner in which this attraction of the molecules for caloric is 

 exerted, there is nothing to decide us a priori in favour of the one of 

 these hypotheses rather than the other ; and we should rather be in- 

 clined to adopt a neutral hypothesis, which would make the distance 

 between the molecules and the quantities of caloric vary according to 

 unknown laws, were it not that the hypothesis we have just proposed 

 is based on that simpHcity of relation between the volumes of gases 



