HISTORY OF THE ATOMIC THEORY. 1 73 



of which it is composed. From this consideration of the 

 attractive forces which tend to form molecules, and of the 

 atmospheres which, in compound elastic fluids, encompass the 

 molecules, but not the ultimate parts severally, we derive an 

 easy explanation of the phenomenon so often noted in the 

 preceding pages — I mean, the conversion of a substance, not 

 into air, but into two or three diflferent elastic fluids, by mere 

 ignition," 



Dr. Higgins thinks of atoms, of simple particles, and even 

 speaks of gases uniting, in some cases, in nearly, if not accu- 

 rately, a fixed proportion, and yet he sees no law. He does 

 not carry his idea far enough. If the molecules are formed by 

 the union of two particles the proportions must of necessity 

 be fixed, or if any number of particles unite, the proportions 

 are fixed, unless the molecules are to be supposed of different 

 constitutions. In this last case, they would constitute a 

 mixture of different gases. But it was not known at this 

 period that all bodies had a fixed constitution, otherwise one 

 would have supposed that Dalton's laws would have been 

 readily arrived at by Dr. Higgins. On the other hand, his 

 theory was not clear, or he would have been led by it to 

 decide on the necessity of fixed constitution as a result, 

 But we obtain no results affecting chemical philosophy. 

 And yet amongst other questions which he says " we shall 

 find no difficulty in answering," is, " Why does any excessive 

 quantity of empyreal or of inflammable air, beyond the deter- 

 minate proportions in which their gross parts can combine, 

 remain elastic and unaltered, or not altered in any considerable 

 part of it, after the combustion ?" * 



The nearest answer he gives as to the proportions is 

 the following : — " The matter of fire limits the quantity in 

 which aeriform fluids, and bodies containing it, can combine 

 chemically." t We may conclude then that nothing in the 

 gases themselves determined the proportion, but the cause 

 was in " distinct atmosphere of fiery matter." 

 • Page 830. f P«»g« 307. 



