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



penetrability, mobility, and vis inertia of the parts ; and thence 

 we conclude that all the least parts of all bodies are extended, 

 hard, impenetrable, moveable, and endued with vis inertia. And 

 this is the foundation of all philosophy." Now, although there 

 is much here to which a modern metaphysician would object, 

 and with good reason, the main idea running through the 

 whole is undoubtedly true, and of the utmost scientific import- 

 ance, viz. that the experience of the senses relative to masses is 

 necessary and sufficient for revealing to us the universal proper- 

 ties of the ultimate constituents of the masses. This doctrine 

 seems to have been very firmly apprehended in the Newtonian 

 epoch of philosophy. Locke has expressed it in more precise 

 terms than those of Newton. In the ' Essay on the Human 

 Understanding'' (Book 2, Chap. 4, § 1) he says, "The idea of 

 solidity we receive by our touch .... There is no idea which 

 we receive more constantly from sensation than solidity " Then, 

 after saying that he does not object to the term impenetrability, 

 but prefers solidity as being in common use to express the same 

 idea, he proceeds to say, "This of all other seems the idea 

 most intimately connected with and essential to body ; so as no- 

 where else to be found or imagined but only in matter. And 

 though our senses take no notice of it but in masses of matter 

 of a bulk to cause a sensation in us, yet the mind having once 

 got this idea from such grosser sensible bodies, traces it further, 

 and considers it, as well as figure, in the minutest particle of 

 matter that can exist ; and finds it inseparably inherent in body 

 wherever and however modified." Again in § 4 he draws a 

 distinction between solidity and hardness, considering the latter 

 to consist in a firm cohesion of parts of matter constituting a 

 mass susceptible of change of figure. According to this defini- 

 tion, hardness, as admitting of degrees, cannot be ranked among 

 Newton's universal properties susceptible of neither increase nor 

 diminution. In fact, absolute hardness is not an idea distin- 

 guishable from impenetrability or solidity. 



In the same Regula III. Newton admits that contiguous parts 

 of bodies are separable, and that undivided parts may in the 

 applications of mathematical reasoning be conceived to be divi- 

 sible in infinitum. But he considers it to be uncertain whether 

 distinct and hitherto undivided parts can be separated by the 

 forces of nature. This doubt has been removed by the progress 

 of chemical science, it being found that after any amount of syn- 

 thesis and analysis the properties of the same portion of a sim- 

 ple body remain the same; which is, at least, presumptive 

 evidence that it consists of parts which do not admit of being 

 divided by natural forces. Thus the least parts of bodies are 

 properly called atoms. There is also another point which expe- 



