Boscovich. 
(620.) 
Dalton’s 
views on 
the subject. 
936 
writers of the recent German school. Abstracting 
as far as possible from the more purely metaphysical 
difficulties (such as those which the consideration of 
Leibnitz’s law of continuity introduces), we may 
perhaps be justified in stating that, whilst the objec- 
tions urged against the existence of atoms fall upon 
our inability to conceive and describe the properties 
of these individual ultimate parts in a consistent 
manner,—the objections to the other notion meet us 
at an earlier stage, and seem to defy any clear con- 
ception of the nature or possible existence of com- 
pound bodies, or of bodies in two states of consis- 
tence. Admitting atoms—whilst acknowledging our 
inability to describe them individually—we can clearly 
enough conceive the phenomena of condensation, ra- 
refaction, evaporation, &c., and also of the combina- 
tion of elements in compounds possessing distinct 
properties; but excluding them, or allowing that 
matter is penetrable by matter throughout, is it pos- 
sible to conceive clearly of such a compound,—as for 
example, of the perfect diffusion of two gases in the 
same space, yet each gas retaining its individuality 
so completely as to admit of easy and complete se- 
paration from the other? The theory of Boscovich, 
which has been sufficiently touched upon by Sir John 
Leslie in the previous Dissertation, was intended, no 
doubt, to reconcile the two opposing theories, and it 
cannot be doubted that it is in many respects an in- 
genious solution. Yet it is essentially (as Professor 
Robison maintains) a corpuscular or atomic doctrine, 
and it farther appears to be diflicultly reconcilable to 
the doctrine of inertia ; for how can a finite number of 
unextended physical points, though they may be the 
centres of intense forces, constitute a finite aggregate 
mass? Nevertheless this speculation has been on 
the whole favourably received, and in our own time 
seems to have been adopted by so eminently practical 
a philosopher as Dr Faraday. 
About Dalton’s idea of atoms, however, there can 
be no doubt. They are, according to his view, pon- 
derable, indivisible masses, having length, breadth, 
and thickness, consequently form. He had distinct 
conceptions of their relative weights, distances, and 
specific heats. He was particularly fond of depicting 
them in diagrams, to which he often refers for a clearer 
exposition of his views than he chose to give in words, 
A mechanical mixture of gases like the atmosphere is 
for him a uniform diffusion of the atoms of each gas 
throughout the space occupied by the whole, and 
MATHEMATICAL AND PHYSICAL SCIENCE. 
* [Dissi VI. 
without reference to the position of the atoms of any 
of the others. But a chemical compound consists 
of molecules or complex atoms, each composed of 
two or more ultimate particles of the constituents 
firmly united by a chemical force, and these complex 
molecules act towards one another exactly as simple 
ones might do. The general notion of chemical forces 
or afinities (as they were perhaps first called by Geoff- 
roy, a French chemist) appears to have been appre- 
hended in two different senses corresponding to the 
atomic or non-atomic theory of body. For the former 
proceeds on the assumption of direct attractions or 
repulsions (push-and-pull forces, as they have been 
called) uniting some and tending to separate others, 
thus assimilating completely chemical with mechani- 
eal forces. The other school adopts the word afinity 
as expressing a mode of action of matter upon matter 
totally distinct from that of force producing motion 
in its particles, to which it is difficult to give an in- 
telligible form, much more to prove that the assump- 
tion is warranted by the facts In this state of 
matters the choice between an opinion perhaps erro- 
neous, and one which assumes no definite shape, can 
hardly, to the practical philosopher, remain long 
doubtful ; when new facts shall have enabled him 
to express intelligibly a new hypothesis, it will be 
time enough to adopt it. 
Dalton s Atomic Theory.—I shall first briefly state 
the general facts or laws to which Dalton gave an uni- 
versal application, and then briefly refer to the un- 
doubted anticipation of part of them by earlier chemists. 
For the sake of distinctness the facts of the atomic theory 
may be thus enumerated :—1. That when two bodies 
unite, not merely by mechanical mixture but through 
a chemical affinity of the elements,—two or more in- 
gredientsforming a whole with new properties,—these 
ingredients are invariably found to exist in constant 
proportions. For instance, the carbonate of lime in- 
variably consists of 44 parts by weight of carbonic 
acid and 56 of lime, the slightest addition of either 
element remaining uncombined, or only mechanically 
mixed with the chemical product. 2. In many 
instances, however, more than one chemical combi- 
nation can be formed between two or more elements, 
and in the simplest cases, where the elements are 
two in number and one remains constant in quantity 
whilst the other increases in amount, a fresh che- 
mical union of the particles does not occur until one 
of the ingredients reaches precisely double the amount 
and in Whewell’s Philos. of Inductive Sciences, vol. i. 
Newton’s conjecture is expressed in these words :— All things considered, 
it seems probable that God, in the beginning, formed matter in solid, massy, hard, impenetrable, moveable particles, of such 
sizes, figures, and with such other properties, and in such proportion to space, as most conduced to the end for which he formed 
them. And that these primitive particles being solids, are incomparably harder than any bodies compounded of them; even 
so very hard as never to wear or break to pieces; no ordinary power being able to divide what God himself made one in the 
first creation.” Horsley’s Newton, vol. iv., 260, quoted by Daubeny. 
1 A profound and subtle thinker of our own time (Mr Leslie Ellis, of Trinity College, Cambridge) has made a definite suggestion 
as to a possible form of chemical forces, viz., that they may not be such as are directly exerted between a particle A and a particle 
B, but only by their presence enable A to act on B, or bear the same relation to force (common force) as force in mechanics does 
to the motion which it causes. Thus, the science of mechanics would include—lst, Kinematics, or pure motion depending on 
equations of the first order ; 2d, Dynamics, depending on equations of the second order ; 3d, Chemi 
ing on equations of higher orders. Camb. Trans., viii., 604, &c. 
, Vital, &c., forces, depend- 
(621.) 
Dalton’s 
laws of che- 
mical com- 
bination. 
