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 binary 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, guoad analysis ; and the physicist is not at liberty 
to convert this negative and relative simplicity into an absolute one, and make 
deductions therefrom, as WoLLaston has done. If we argue from analogy, in re- 
ference to this point, we 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 asingle 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 purpose to decline 
assent to Wortasron’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 probandi 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 argumentum ad ignorantiam. He was 
guilty also of a petitio precip. For even, if it could be shewn, that oxygen and 
nitrogen are chemically homogeneous, and do not, on that account, 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 greater 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 supplied 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. 
