502 Prof. J. Lovering on the Mathematical and 



In 1869 and 1873, Prof. Challis of Cambridge, England, pub- 

 lished two works on the Principles of Mathematical Physics. 

 They embody the mature reflections of a mathematical physicist 

 at the advanced age of three score years and ten. Challis be- 

 lieves that there is sufficient evidence for the existence of aether 

 and atoms as physical realities. He then proceeds to say : — "The 

 fundamental and only admissible idea of force is that of pressure, 

 exerted either actively by the aether against the surface of the 

 atoms, or as reaction of the atoms on the aether by resistance to 

 that pressure. The principle of deriving fundamental physical 

 conceptions from the indications of the senses does not admit of 

 regarding gravity, or any other force varying with distance, as 

 an essential quality of matter, because, according to that prin- 

 ciple, we must, in seeking for the simplest idea of physical force, 

 have regard to the sense of touch. Now, by this sense we ob- 

 tain a perception of force as pressure, distinct and unique, and 

 not involving the variable element of distance, which enters into 

 the perception of force as derived from the sense of sight alone. 

 Thus, on the ground of simplicity as well as of distinct percepti- 

 bility, the fundamental idea of force is pressure/' As all other 

 matter is passive except when acted upon by the aether, the aether 

 itself, in its quiescent state, must have uniform density. It must 

 be coextensive with the vast regions in which material force is 

 displayed. Challis had prepared himself for the elucidation and 

 defence of his dynamical theory by a profound study of the laws 

 of motion in elastic fluids. From the mathematical forms in 

 which he has expressed these laws he has attempted to derive 

 the principal experimental results in light, heat, gravitation, 

 electricity, and magnetism. Some may think that Mr. Challis 

 has done nothing but clothe his theory in the cast-off* garments 

 of an obsolete philosophy. If its dress is old, it walks upon new 

 legs. The interplay between aether and atoms is now brought on 

 to the stage, not as a speculation sustained by metaphysical and 

 theological arguments, but as a physical reality with mathema- 

 tical supports. I should do great injustice to this author if I left 

 the impression that he himself claimed to have covered the whole 

 ground of his system by proof. Mathematical difficulties pre- 

 vented him from reaching a numerical value for the resultant 

 action of a wave of aether upon the atom. What he has written 

 is the guide-post, pointing the direction in which science is next 

 to travel; but the end of the journey is yet a great way off. 

 The repeated protests of Mr. Challis against the popular physics 

 of the day, and his bold proclamation of the native, independent 

 motion of the aether, have aroused criticism. What prevents the 

 free aether, asks the late Sir John Herschel, from expanding into 

 infinite space? Mr. Challis replies that we know nothing about 



