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philosophy." Against this dictum the author argues as follows 

 (p. 27) : " Our first impressions of matter are derived from solids, 

 and their resistance to touch, or pressure of the hand. Hence the 

 nature of matter has often been defined by extension and solidity. 

 But the progress of science has shown the error of such a definition. 

 The sensation of solidity is evidently compound, and arises from a 

 repulsive force exerted along a well-defined surface. This repulsion 

 begins before actual contact, and appears to be a rapidly decreasing 

 power, which emanates from the outer particles of the resisting sub- 

 stance. When it is melted or vaporized, the substance remains, but 

 its law of force is altered, and the sensation of solidity disappears. 

 Again, the impenetrability of matter, in the popular sense, is dis- 

 proved by the facts of chemical combination." .The answer to all 

 this is, that the arguments are directed against non-essentials in 

 Newton's definition of the " least parts of bodies." The point 

 of primary importance is that, as all experience shows, we have no 

 perception by the senses of body, and, therefore, no conception of 

 bod3 r , apart from extension. On this point metaphysics and physics, 

 Locke and Newton, agree. But if the ultimate parts of bodies have 

 extension, they must be solids (in the geometrical sense), and if they 

 are solids, they must have/orm. Here, however, so much as this 

 may be conceded, that the terms " hardness " and " impenetrability " 

 are open to the objection made by Mr. Birks that, as these qualities 

 are understood by experience, they admit of degrees. But this ob- 

 jection is met, and at the same time the definition in all that is es- 

 sential is retained, if we say that an ultimate particle which has form, 

 has a constant form. This hypothetical property of an atom is to be 

 adopted on the principle, laid down by Mr. Birks himself in his 

 Axiom II. (p. 4), that it is the " simplest" conceivable. The varia- 

 bility of form exhibited by masses is shown by the progress of 

 science to be due to their being composed of discrete atoms, and it 

 would therefore be introducing a needless complexity to attribute 

 the same variability to the atoms. 



In short the grand and leading principle of the Newtonian Philo- 

 sophy is to deduce by means of mathematics from the fundamental 

 ideas of form, inertia, and pressure, results by which observed phe- 

 nomena and laws may be explained, the data for the solution of the 

 particular problems being also furnished by experimental observation. 

 If then the philosophy which Newton taught and endeavoured to 

 hand down to succeeding times be true, and be truly represented 

 above, works such as that before us must contain a kind of scientific 

 heresy. For not only have we here direct antagonism in defining a 

 monad, or atom, to be a centre of force irrespective of the quality of 

 extension, but this first departure from the simplicity of Newton's 

 fundamental ideas necessitates other complexities, and gives rise 

 to the conceptions of axes, rotations, polarities, and centrifugal 

 force of atoms, which play a large part in the author's system, and 

 to which effects are ascribed that seem incapable of illustration by 

 reference to commonly observed phenomena. On the contrary, the 

 Newtonian " foundation of all philosophy " involves only ideas that 



