84 DR GEORGE WILSON ON THE FINITE DIVISIBILITY OF MATTER. 



atoms, one of hydrogen and one of oxygen, with a centre of repulsion common to 

 both. The molecule of carbonic acid, for similar reasons, would consist of three 

 atoms, one of carbon and two of oxygen. And who shall assure us that oxygen 

 and nitrogen are not compounds with binai-y chemical molecules like those of 

 water, or ternary ones like those of carbonic acid ? It is true that the chemist 

 names these gases simple substances. But the simplicity he attributes to them 

 is only, as he is careful to define, quoad anah/sis ; and the physicist is not at liberty 

 to convert this negative and relative simplicity into an absolute one, and make 

 deductions therefrom, as Woliaston has done. If we argue from analogy, in re- 

 ference to this point, \ve should infer that oxygen and nitrogen are compounds ; 

 for we know a much greater number of gases in which the molecule is a group of 

 chemically distinct atoms, than we do of elastic fluids, where, on the most favour- 

 able view, it can be supposed to be a single one. But it is not necessary to pursue 

 any argument of this kind ; nor is the objector called upon to shew that oxygen 

 and nitrogen are chemical compounds. It is sufficient for his puri)ose to decline 

 assent to Wollaston's conclusion till he, or those who agree with him, supply 

 proof that the molecules of oxygen and of nitrogen are chemically simple. The 

 onus prohandi clearly lies, not with the denier but with the asserter, of a positive 

 proposition like the one before us. 



In so far, then, as Wollaston assumed the chemical simplicity of two of the 

 gases of the atmosphere, he employed an ar<ji(nientwm ad v/norantiam. He was 

 guilty also of a petitio principii. For even, if it could be shewn, that oxygen and 

 nitrogen are chemically homogeneous, and do not, on that accoimt, admit of com- 

 parison as to the constitution of their gaseous molecule with water and carbonic 

 acid, it would not warrant the conclusion, that that molecule was an atom. Does 

 it follow as a necessary inference, that because a body is simple, its gaseous repel- 

 ling molecule must consist of but one atom ? The answer is assuredly in the ne- 

 gative. The molecule might, on the other hand, be made up of a pair of atoms, 

 like a binary star, with a centre of repulsion common to the two ; or of 10, or of 100, 

 or 1000 atoms (if such bodies there be), grouped together into a compound whole. 

 We have no means whatever, in truth, of estimating what the complexity of the 

 molecule may be. Without insisting at gi-eater length on this, it is at least mani- 

 fest, that we are not even at liberty to identify the combining chemical mole- 

 cule with the repelling gaseous one, much less to identify either with the ulti- 

 mate atom. Yet, unless Wollaston was at liberty to do so, his argument was 

 useless towards settling the question of the divisibility of matter. To prove that 

 the atmosphere consisted of finite molecules, was only to reach the threshold of 

 the difficulty : for each molecule sujjplied as good a text whereon to dispute the 

 question of infinite divisibility, as the whole atmosphere out of which it was taken. 

 The point which most of all demanded proof, namely, that the molecule was an 

 atom, was the very one which he took for granted. 



