THE MECHANICAL ACTION OF LIGHT. 695 



tioii. By experimenting at this critical pressure, and at the same 

 time taking all the precautions which experience shows are necessary, 

 it would seem that such an action as was obtained by Cavendish, 

 Reich, and Baily, should be rendered evident. 



It is not unlikely that in the 'experiments here recorded may be 

 found the key of some as yet unsolved problems in celestial mechanics. 

 In the sun's radiation passing through the quasi vacuum of space we 

 have the radial repulsive force, possessing successive propagation, re- 

 quired to account for the changes of form in the lighter matter of 

 comets and nebulae, and we may learn by that action, which is rapid 

 and apparently fitful, to find the cause in those rapid bursts which 

 take place in the central body of our system ; but until we measure 

 the force more exactly we shall be unable to say how much influence 

 it may have in keeping the heavenly bodies at their respective dis- 

 tances. 



So far as repulsion is concerned, we may argue from small things 

 to great, from pieces of pith up to heavenly bodies ; and we find that 

 the repulsion shown between a cold and warm body will equally pre- 

 vail, when for melting ice is substituted the cold surface of our at- 

 mospheric sea in space, for a lump of pith a celestial sphere, and for 

 an artificial vacuum a stellar void. 



Throughout the course of these investigations I have endeavored 

 to remain unfettered by the hasty adoption of a theory, which, in the 

 early stages of an inquiry, must almost of necessity be erroneous. 

 Some minds are so constituted that they seem impelled to form a the- 

 ory on the slightest experimental basis. There is then great danger 

 of their becoming advocates, and itnconscioasly favoring facts which 

 seem to prove their preconceived ideas, and neglecting others which 

 might oppose their views. This is unfortunate, for the mind should 

 always be free to exercise the judicial function, and give impartial 

 weight to every phenomenon which is brought it. Any theory will 

 account for some facts ; but only the true explanation will satisfy all 

 the conditions of the problem, and this cannot be said of any theory 

 which has yet come to my mind. 



My object at present is to ascertain facts, varying the conditions 

 of each experiment so as to find out what are the necessary and what 

 the accidental accompaniments of the phenomena. By working stead- 

 ily in this manner, letting each group of experiments point out the 

 direction for the next group, and following up as closely as possible, 

 not only the main line of research, but also the little by-lanes which 

 often lead to the most valuable results, after a time the facts will 

 group themselves together and tell their own tale ; the conditions un- 

 der which the phenomena invariably occur will give the laws ; and 

 the theory will follow without much difficulty. The eloquent lan- 

 guage of Sir Humphry Davy contains valuable advice, although in 

 terms somewhat exaggerated. He says : " When I consider the va- 



