Prof. Challis on Newton's " Foundation of all Philosophy ." 283 



riment seems to have settled. As the atoms of bodies have ex- 

 tension, they must necessarily have form; and the question 

 arises, what is their form ? Since, it is found that the movement 

 of the parts of a fluid body inter se alters in no manner or 

 degree the mutual relations of the parts or the sensible qualities 

 of the body, it seems inevitably to follow that each atom has the 

 same relation to space and to surrounding atoms in all directions 

 from its position, and consequently that its form is spherical. 



Inertia is a universal quality of bodies known from experience to 

 belong to masses ; and on the principle that a universal quality of 

 masses reveals a universal quality of the constituent atoms, we may 

 affirm that each atom is inert. But this property, which Newton 

 calls " vis insita," and affirms to be " immutabilis," does not, in 

 fact, appear from any sensible evidence to be susceptible of mea- 

 sure or degree. There may be more or less of inert matter, but 

 not more or less of inertia. Accordingly all atoms are of the 

 same intrinsic inertia, and differ only in magnitude. 



The fundamental ideas respecting matter, to which the fore- 

 going discussion leads, may be concisely expressed in the follow- 

 ing terms : — All matter consists of inert spherical atoms of dif- 

 ferent, but constant, magnitudes. 



The fundamental ideas respecting force admit of similar inves- 

 tigation. On this head Newton has said but little, but perhaps 

 as much as the state of physical science in his day allowed him 

 to say. With respect to the force of gravity, he has left on record 

 (in Reg. III.) that he by no means considered it to be " essential 

 to bodies," assigning as the reason for not admitting it to be such, 

 that it is mutable as to space, changing with the distance of the 

 attracting body. Also in the very last paragraph of the Priii- 

 cipia he allows of the existence of " a certain very subtle spirit 

 pervading gross bodies and concealed in them ; by the force and 

 agency of which the particles of bodies mutually attract at very 

 small distances, and when contiguous, cohere ; and electric bodies 

 act at greater distances, both repelling and attracting neigh- 

 bouring bodies; and light is emitted, reflected, refracted, in- 

 flected; and bodies are warmed; and all sensation is excited, 

 and the limbs of animals are moved at will, &c." But true to 

 the principle of deriving fundamental ideas from the indications 

 of experience, he refrains from pursuing these views further, 

 because "there was not a sufficient abundance of experiments 

 proper for determining and exhibiting the laws of the action of 

 this fluid." Not only were experiments wanted in Newton's 

 time for such determinations, but the principles of the mathe- 

 matical reasoning by which those laws might be deduced from 

 appropriate experimental data were unknown. 



The case, however, is different in the present state of our 



