138 



MATHEMATICAL AND PHYSICAL SCIENCE. 



[Diss. VI. 



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- 



Boscovich. 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 difficultly 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. 

 (620.) About Dalton's idea of atoms, however, there can 



Dalton's foe no doubt. They are, according to his view, pon- 

 K- n , derable, indivisible masses, having length, breadth, 



SUOJ6CD* i * ' i i / ._ - - ,, . 



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 



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 affinities (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- 

 cal forces. The other school adopts the word affinity 

 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. 1 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 nave enabled him 

 to express intelligibly a new hypothesis, it will be 

 time enough to adopt it. 



Dalian'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- m i ca i coi 

 doubted anticipation of part of them by earlier chemists, bination. 

 For the sake of distinctness the/acte 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- 

 gredients forming 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 



(621.) 



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 1st, Kinematics, or pure motion depending on 

 equations of the first order ; 2d, Dynamics, depending on equations of the second order ; 3d, Chemical, Vital, &c., forces, depend- 

 ing on equations of higher orders. Camb. Trans., viii., 604, &c. 



