208 CHALLIS'S HYDKO-DYNAMIC THEORY 



The dynamic theory of gaseous elasticity, diffusion, and 

 pressure opens the way for a general conception of the 

 manner in which all the forces may depend on move- 

 ments of the matter or the sether, and I may, therefore, 

 briefly refer to Professor Challis's hydro-dynamic 

 theory* without presuming to give an opinion upon its 

 absolute truth, for even as a mere diagram it would 

 illustrate the subject better than a train of reasoning. 



The hypotheses of his theory are these only : " 1st, All visible 

 and tangible substances consist of inert spherical atoms of con- 

 stant form and magnitude (for each kind) ; 2nd, All physical 

 force is either active pressure of the aether, supposed to be a 

 continuous elastic fluid having the property of pressing always 

 proportionally to its density, or passive resistance of the atoms 

 to such pressure due to their inertia and constancy of form " 

 (104). The existence and qualities of the atoms must be taken 

 as"ultimate facts, and out of the province of a priori investiga- 

 tion (105). Professor Challis thus uses the word force in the 

 sense of the antecedent pressure which produces motion, while, 

 as above said, others think it more convenient to apply it to 

 the motion itself (Grove), and Mayer to " what is expended." 

 But, practically, it comes to the same thing, for all the pheno- 

 mena by which we recognize force are traced by Challis to 

 movements of the aether or the atoms. Thus heat and light 

 are movements of the matter and aBther of a vibratory kind, 

 while electricity, galvanism, and magnetism are similar but of 

 a more complicated character. In these Challis does not differ 

 substantially from other authors. But with respect to the 

 passive forces, viz., the molecular attractions and repulsions 

 which it is impossible to conceive as produced by movements 

 of the atoms themselves, and which have been hitherto the 

 grand stumbling blocks to the full comprehension of the nature 

 of force as a whole, he holds that they are produced by undula- 



* "Essay on the Mathematical Principles of Physics" (Cambridge, 

 1873). 



