Prof. Challis on the Principles of Theoretical Physics. 319 



of taking it for granted in prosecuting further researches, would 

 certainly be a great step in theoretical physics. The other class 

 of hypotheses do not admit of the same kind of verification, 

 because phenomena (such as are some of those of light) which 

 depend on the qualities of the constituents of bodies, require in 

 general for their direct theoretical explanation the knowledge 

 of the mutual action between the aether and the atoms, and the 

 number, magnitudes, and arrangements of the latter. But this 

 knowledge cannot be furnished in the present state of physical 

 science, and ought rather to be looked for as the final result of 

 physical inquiry pursued in different channels and by all avail- 

 able means. 



Such being the account of the hypotheses of the proposed 

 physical theory, I proceed now to speak briefly and in general 

 terms of the course of reasoning required in their application to 

 different classes of phenomena, and the demands which they will 

 have to satisfy. First, let it be conceded that the before-men- 

 tioned explanations of phenomena of light have given strong 

 presumptive evidence of the existence of the aether, and of its 

 being such that variations of its pressure are proportional to 

 variations of its density. Next we must take into account the 

 matter of fact that light-bearing rays are also heat-bearing, and 

 that consequently the aether must be the vehicle of the transmis- 

 sion both of light and of heat. The explanation which the undu- 

 latory theory gives at once of this fact is, that in a ray there exist 

 conjointly transverse and direct vibrations, and that the former 

 expound light, and the latter heat. Again, as heat is known by 

 experience to act as a repulsive force, the aether which accounts 

 for other of its phenomena must account for this also, It must 

 be borne in mind that the aether was assumed to be a highly 

 elastic medium, and its dynamic action cannot therefore be over- 

 looked. In fact it is reasonable to attribute the sensation of 

 light to the dynamic effect of the setherial vibrations on the ner- 

 vous system of the eye. But such vibrations, when we calculate 

 their effect only to the first power of the velocity, are found to 

 produce simply oscillations of small spherical bodies submitted 

 to their action, and not motions of translation. To account for 

 the latter, it is necessary to proceed to the consideration of effects 

 due to the second power of the velocity. This I have attempted 

 to do, and to found a theory of the force of heat on a mathema- 

 tical investigation of the dynamic effect of pressures correspond- 

 ing to the square of the velocity in setherial vibrations. The 

 investigation showed that the result of such action on small 

 spherical bodies might under some circumstances be repulsion 

 from a centre, under others, attraction towards a centre. Thus 

 the theory was found to embrace the forces of aggregation which 



