466 Prof. Challis on the Fundamental Ideas of 



going statement of them with the accompanying remarks may 

 suffice to show that they are such as to satisfy the conditions 

 which, as before said, are requisites of the hypotheses of a phy- 

 sical theory. The opinion generally entertained that gravity is 

 an essential property of matter forms no part of the theory. It 

 is a gratuitous speculation without physical significance. 



Before dismissing the subject of gravity, another remark may 

 be appropriately made, bearing upon the relation of the force of 

 gravity to a general physical theory. Assuming, as the present 

 state of physical astronomy allows us to do, that the Theory of 

 Gravitation has been established by the combination of calcula- 

 tion with observation, it will follow that the hypotheses of the 

 theory are proved to be physical realities. But the terms in 

 which they are expressed sufficiently show that they are not 

 ultimate facts, but rather laws, and as such deducible by mathe- 

 matical reasoning from the fundamental ideas of matter and 

 force, it being a rule of philosophy that all quantitative laws 

 admit of a priori deduction from initial principles. The hypo- 

 theses of gravitation being proved to be true, we might reason- 

 ably ask the questions, Why is the attracting power of a mass 

 proportional to the mass ? Why does it vary inversely as the 

 square of the distance from the mass ? What is the reason that 

 the attraction of gravity adds in every small given interval of 

 time an increment of velocity in the direction in which it acts 

 proportional to the attracting power, independently of the size, 

 density, and actual motion of the attracted body ? Why is there 

 no interference of different attractions acting at the same time 

 on the same body ? and how is it that the attractive power pene- 

 trates into the substances of bodies and passes through them 

 with no perceptible change as to law or amount ? These are 

 all questions to which answers may be expected to be given in 

 the progress of physical, science; and it would be the special 

 province of a general physical theory founded on the funda- 

 mental ideas of matter and force to answer by the aid of mathe- 

 matics as well these questions relating to the force of gravity, as 

 similar questions relating to the other physical forces. This 

 brings me to the consideration of the hypotheses of my general 

 Physical Theory already enunciated, which I now proceed to 

 discuss one by one, with the view of showing that they are 

 framed in accordance with the three conditions which the foun- 

 dations of an intelligible physical theory are required to satisfy. 



(1) Matter is not infinitely divisible like space; for then it 

 would not be distinguishable from space. All bodies, therefore, 

 have parts that are not separable into other parts ; that is, they 

 have ultimate parts that may be named atoms. The hypothesis 

 of atomic constitution is supported by chemical analysis and 



