132 Letter to the Editor, by Dr. Henry. 
gravitation itself, however firmly it may now be established, took its rise in 
an hypothesis founded on analogy, and could be considered as nothing more 
than an hypothesis, till that period of the life of its great author, when the 
coincidence was ascertained between the law which regulates the fall of 
heavy bodies, and that power which preserves the moon in her orbit.‘ A 
principle,” it has been remarked by the late Professor Playfair, “is often 
admitted in physics, merely because it explains a great number of appear- 
ances, and the theory of gravitation itself rests on no other foundation*.” 
The term hypothesis, then, is far from being one of just reproach, since it 
may be applied in a variety of cases to those first steps which it has been 
found necessary to take in philosophical inquiries, and which have led 
eventually to well-established laws. 
The views of Mr. Dalton respecting the atomic constitution of bodies 
appear to me to be founded mainly on the general fact, that bodies unite in 
definite proportions. Of this general truth, Richter certainly furnished 
the best and fullest evidences. Far from wishing to ‘‘ suppress” the share 
of credit to whicli he is entitled, I have alluded to the table, calculated by 
Fischer from his experiments ; but it is omitted in the appendix to the pre- 
sent edition, merely because it has been superseded by the more extensive 
tables of equivalents, which have since been constructed. ‘The law of com- 
bination ia multiple proportions, the first experimental proofs of which are 
due to Mr. Dalton, comes strongly in aid of the atomic theory, and fur- 
nishes its most striking proofs and illustrations. Nothing can be more 
evident than that if we set out from a binary compound, whose gaseous 
elements exist in equal volumes, and proceed to compounds of the same 
elements, in which either is found as a multiple in volume of the other, there 
must, as the reviewer observes, ‘‘ be a perfect accordance between the ato- 
mic hypothesis and the theory of volumes.” But the atomic theory is, [ 
contend, a wider and more comprehensive generalization, and includes the 
Jaw of volumes as well as that of combination by multiples of weights. In 
this ease, as in many others, when we advance from discovery to discovery, 
ave do nothing more than resolve our former. conclusions into others still 
gore general. 
There can surely be nothing inconsistent with sound philosophy in in- 
quiring why bodies unite in definite proportions, and why they unite in pro- 
portions which are multiples or sub-multiples of weights or oF volumes ; and 
the only satisfactory explanation, that has yet been given of these 
facts is, that in those combining weights, which are represented by 
equivalent numbers, are contained determinate numbers of ultimate particles 
or atoms, and that from the relative weights of aggregates that combine, we 
may deduce the proportions as to weight which the ultimate single atoms bear 
to each other. As there-seems every reason to believe that chemical attrac- 
tion is exerted, not between masses, but between ultimate particles or atoms 
only, combination will then take place either between single atoms or when 
either is in excess, the excess will be represented by some simple multiple of 
the number of atoms. In this reasoning it is of course taken for granted that 
matter is not infinitely divisible, a position rendered extremely probable by 
‘a philosopher, to whose opinions the reviewer will agree with me in paying 
the greatest deference. ‘‘ Now though we have not the means,” that 
writer observes, ‘of ascertaining the extent of our own atmosphere, those 
of other planetary bodies are nevertheless objects for astronomical investi- 
* Playfair’s Works, vol. iy, p, 62, note. 
