KINETIC THEORIES OF GRAVITATION. 251 



seek the cause of this pressure, it forever eludes us. Here tbeu the 

 system stands self-convicted of impotence to exercise its prime preroga- 

 tive. At whatever point in the infinite series of descending orders of 

 lether we stop, the secret of its power is ever one step backward. We 

 must still "conceive of the existence of another order of nsther having 

 the same relation to it'^ that it had to the preceding. And that no pos- 

 sible, element of embarrassment may be wanting to our conception, the 

 first tether is absolutely " continuous," without atoms and without in- 

 terstices ! 



In 1SG9, Professor Challis published an elaborate extension of his 

 mathematical discussion of kinetic theories of the physical forces, in a 

 large octavo volume of some 750 pages ; the first half of the work being 

 devoted to a general mathematical treatise, of high merit and value, 

 under the title of " Notes on the Principles of Pure and Applied Calcu- 

 lation." In the latter portion of the work, (on theoretical physics,) the 

 subjects treated of" are those of light, heat, and molecular attraction, 

 force of gravity, electricity, galvanism, and magnetism, respecting which 

 I make the general hypothesis that their phenomena all result from 

 modes of action of an elastic fluid, the pressure of which is proportional 

 to its density."* And in the " introduction " to the work, he has more 

 explicitly stated: "The hypothesis respecting the tether is simply that 

 it is a continuous elastic medium, perfectly fluid, and I hat it presses pro- 

 portionally to its density." t The forces of elasticity, and of chemical 

 affinity, are excluded as beyond the present reach of analysis. 



A distinction is made between the jetherial radiations of light and of 

 heat, not justified by any observed phenomenon. " Since in the theory 

 I have proposed, the transverse vibrations of rays always accompany 

 direct vibrations, and it was concluded that the sensation of light is en- 

 tirely due to the former, we are at liberty to refer the action of heat, or 

 other modes of force, to the direct vibrations." | This would leave the 

 polarization of heat quite inexplicable ; as obviously vibrations of the 

 acoustic type are incapable of polarization. 



It is now familiar to opticians that fine rulings on glass, whose dis- 

 tance apart is less than a half of the wave-length of light, are readily 

 resolvable with optical distinctness by our modern microscopes, while 

 the intimate texture of the glass is apparently as far removed from res- 

 olution as with the unarmed eye. What part can petherial vibrations 

 l)l,ay in giving cohesion to the ultimate molecules of the glass? Here 

 then is apparently a new difiiculty for the undulatory theory of force; 

 for not only are the ultimate molecules of the silicate bound together 

 with a powerful force,, (giving seeming continuity of substance to our 

 highest artificial vision,) but they are also held apart with a still more 

 potent stress. Professor Challis does not shrink from the solution. 



* Principles of Mathematics aud Physics, bvo., Cambridge, 1669. p. 31d. 

 tlutrocluctiou, p. xlv 

 t O^cre ciiat., p. 437. 



