64 SCIENCE IN SHORT CHAPTERS. 



the universe, and all their attendant worlds, comets, and 

 meteors. 



In order to prevent misunderstanding of the above, I 

 must add that I have therein studiously assumed a negative 

 position in reference to all hypothetical conceptions of the 

 nature of heat, light, etc., and their modes of transmission, 

 simply because I feel satisfied that the subject has hitherto 

 been obscured and complicated by overstrained efforts to fit 

 the phenomena to the excessively definite hypotheses of 

 modern molecular mathematicians. The atoms invented by 

 Dalton for the purpose of explaining the demonstrated laws 

 of chemical combination performed this function admirably, 

 and had great educational value, so long as their purely 

 imaginary origin was kept in view; but when such atoms are 

 treated as facts, and physical dogmas are based upon the 

 assumption of their actual existence, they become dangerous 

 physical superstitions. Regarding matter as continuous, 

 i. e., supposing it to be simply as it appears to be, and co- 

 extensive with the universe, in accordance with the experi- 

 mental evidences of the unlimited expansibility of gaseous 

 matter, we need only assume that our sensations of heat, 

 light, etc., are produced by active conditions of such mat- 

 ter analogous to those which are proved to produce our sen- 

 sations of sound. On this basis there is no difficulty in 

 conceiving the rationale of the reaction which produces the 

 repulsion of the radiometer. I may even go further, and 

 affirm that it is impossible to rationally conceive radiation 

 producing any mechanical effects without mechanical reac- 

 tion. If heat be motion,, and actual motion of actual 

 matter, mechanical force must be exerted to produce it, and 

 a body which is warmer on one side than the other, i. e., 

 which is exerting more outward motion-producing force on 

 one side than on the other, must be subject to proportionally 

 unequal reaction, and, therefore, if free to move, must re- 

 treat in a direction contrary to that of its greater activity. 

 Regarded thus, the residual air of the radiometer does act, 

 not by collisions of particles between the vane and inside of 

 the glass vessel, but by the direct reaction of the radiant 

 energy which would operate irrespective of vessels, i. e., 

 upon naked radiometer vanes if carried halfway to the 



