492 THE CONSERVATION OP ENERGY 



modes of action., without the intervention of a material connection 

 between the two ; the other class, on the conti-ary, maintains that this 

 is impossible and inconceivable, and endeavours, from the hypothesis 

 of a material connection, to show that the observed laws necessaiily 

 follow from the essential properties of matter. 



These theories are known respectively as the doctrines of action at 

 a distance, and action by contact. 



In order to compare these theories, it will be necessary to fix upon 

 some possible arrangement of the universe to which both classes of 

 thinkers may yield assent, and then decide upon the merits of each 

 of the above doctrines by applying them in explanation of natural 

 phenomena, or by testing their consistency with inductions from 

 natural phenomena other than the laws of attraction and repulsion. 



Imagine then material bodies to consist of very minute indivisible 

 particles of continuous matter, separated by minute intervals of space ; 

 also, suppose that all these bodies are surrounded and saturated, so 

 to speak, with a highly elastic medium of extreme tenuity, which 

 itself consists of much more minute particles of matter, separated by 

 empty spaces. The first-mentioned matter corresjionds to what may 

 be called gross matter ; the second, to the luminiferous ether. It is 

 true that there is a conception of the constitution of matter which 

 at first sight may seem to be different from the above, and which of 

 late years has found great acceptance, viz., the notion that an atom 

 is a vortex ring in a frictionless fluid ; but the arrangement above 

 assumed may include this, as the vortex atoms, if such there be, may 

 be constructed out of the assumed material. 



In accordance with the theory of action at a distance, the above 

 particles of matter all act on each other by means of certain attractions 

 and repulsions with which they are indued, and also by collision. 



In the action by contact theory, the various attractions, repulsions, 

 elasticities,' &c., are referred solely to the motions of the particles, 

 and their collisions. The touchstone by which we intend to test the 

 rival theories, is the principle of the conservation of energy. It is 

 true that observation has not completely established the truth of this 

 principle as a law of nature, but the conviction of its truth is 

 becoming stronger day by day as our observations increase ; and no 

 theory of the constitution and modes of action (or forces) of matter 

 can be considered as likely to be true, which can be shown to be 

 inconsistent with this principle. 



