709 
TIIE CONSTITUTION OF BODY, ETC. 
BY AN ATOM. 
As Mr. Tilclen is good enough to consider my sketchy papers worth his notice, 
I trust that I may not be pushing the matter too far if I endeavour, very 
briefly, to clear np those points on which we virtually agree, and to bring into 
stronger relief those upon which we still differ ; these last being chiefly, the 
expediency (for I am gratified to find that he recognizes the propriety) of drop¬ 
ping the use of the word matter in chemical inquiries ; the problem which I 
have stated as ( x ) ; and the relation of heat, light, etc., to motion. 
I follow his observations in the order of their occurrence. 
1. I have not maintained, as Mr. Tilden appears to suppose, that the fate of 
chemistry hinges upon that of the, or even of any particular atomic theory. 
What I said was, that an atomic theory was an absolutely indispensable hypo¬ 
thesis, etc. I should even be far from holding that the present version of that 
theory is either complete or final. For, though it suffices to explain the ma¬ 
jority of facts which have hitherto been brought under observation, and, if libe¬ 
rally interpreted, would probably account for all, it is quite possible that, like 
the Ptolemaic system of astronomy, which, with its mazy complexity,— u cycle 
in epicycle, orb on orb,”—still managed to explain and account for the move¬ 
ments of the heavenly bodies, atoning the original falsity of its stand-point by 
a most ingenious system of compensations and adaptations ; it, too, may some 
day have to give place to a more Copernican theory, which shall command the 
immediate assent of all. The essential principle of any atomic theory is the 
absolute finity of physical parts; and these chemistry is compelled to accept as 
her first data—the primar} 7 elements with which she has to deal; but into their 
nature, unless as matter of curiosity, she has no more need to inquire than 
the recipient has to look a gift horse in the mouth. 
2. In styling chemistry a science of precision, I spoke of the intrinsic nature 
of the science, not of its present condition, which, as Mr. Tilden observes, was 
subsequently characterized as that of childhood. Just as, before Thales, one 
would still have been justified in terming geometry a science of precision, 
though, up to that time, its condition was only such as its name expresses, viz. 
an art of mensuration. 
3. Undoubtedly Mr. Tilden only employs the term matter in the sense com¬ 
mon with nearly, or quite, all physicists ; nor did my remarks individualize him 
as doing anything exceptional. But he will pardon me for saying, that I cannot 
see that chemistry has anything whatever to do with matter, which is a word 
that, unless when opposed to force, should never be heard within its precincts. 
It is true that body is material; but so also is it telluric. But, as we have no 
occasion to call it telluric, so neither need we call it material. The word, in its 
true sense, is mere surplusage in the science, and of no value whatever. As he 
is good enough to approve of the applications to which the terms matter, sub¬ 
stance, and body ought to be restricted, I wish he could be induced to set the 
example of so restricting them, if only to a slight extent. He objects to the 
difficulty. But such a difficulty is like that of erecting a piece of machinery,— 
the useful work accomplished by which, when once it has been set up, trans¬ 
cends indefinitely the limited amount of labour expended, once for all, in erect¬ 
ing it. 
4. In my slight remarks upou causes, I did not intend any special reference 
to Mr. Tilden’s employment of the word. I would observe, however, upon his 
present use of the term, that the cause of which he now speaks is that which 
has very pertinently been termed the occasional cause. For instance, given 
certain conditions, a, &, c, all of which have been duly observed; then add 
