Matter and Force in Theoretical Physics. 467 



synthesis, and by the facts that bodies may be broken, may 

 change as to form by alteration of the relative positions of the 

 parts, and may change as to magnitude by compression or dila- 

 tation. The last fact implies that the atoms are separated by 

 spatial intervals. 



(2) The essential qualities of matter are those we perceive by 

 the senses. This is Newton's fundamental principle. All bodies 

 of which we take cognizance by the senses have form. There- 

 fore the atoms, or ultimate constituents of bodies, have form ; 

 otherwise they want an essential quality of matter. Change of 

 form, which we perceive bodies to be capable of, is a consequence 

 of atomic constitution, and therefore cannot be predicated of the 

 atoms themselves. 



(3) All that is said in paragraph (2) is equally true if magni- 

 tude be substituted for form. 



(4) We know what inertia is by experience of it in masses, 

 and it is found by experiment to be always the same in the same 

 quantity of matter independently of all other properties. Hence 

 inertia is an essential quality of matter, sui generis, existing in 

 masses only because it exists in their ultimate parts. Thus the 

 atoms of bodies are essentially inert. 



(5) In order to reason mathematically on the ultimate parts 

 of bodies, it is necessary to ascribe to them a particular form. 

 The spherical form is suggested by observation and experiment, 

 it being found that however a given mass may be turned about, 

 the direction of the earth's attraction remaining the same, how- 

 ever the different parts may change their relative positions, 

 whether the mass be in the solid, fluid, or gaseous state, and 

 whatever analysis or synthesis it may undergo, the action of the 

 earth's gravity upon the whole, and therefore upon each atom 

 (that is, the weight of the whole and of each atom), are constantly 

 the same. These facts point to a spherical form as sufficiently 

 accounting for the indifference of the atom as to the direction 

 of the external action, and as to its being exerted under differ- 

 ent physical circumstances. This form may therefore be taken 

 hypothetically as the basis of mathematical reasoning applied to 

 atoms. 



(6) There is no other kind of force than pressure by contact 

 of one body with another. This hypothesis is made on the prin- 

 ciple of admitting no fundamental ideas that are not referable to 

 sensation and experience. Tt is true that we see bodies obeying 

 the influence of an external force, as when a body descends 

 towards the earth by the action of gravity ; and so far as the 

 sense of sight informs us, we do not in such cases perceive either 

 the contact or the pressure of another body. But we have also 

 the sense of touch and of pressure, by contact, for instance, of the 



