THE PHILOSOPHY OF THE BADIOMETEK. 115 



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 

 is it appears to be, and co-extensive with the universe, in ac- 

 cordance with the experimental evidences of the unlimited 

 expansibility of gaseous matter we need only assume that our 

 sensations of heat, light, etc. are produced by active condi- 

 tions of such matter analogous to those which are proved to 

 produce our sensation 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 

 reaction. 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 retreat in a direc- 

 tion 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 half way to the moon, or otherwise freed from 

 excess of atmospheric embarrassment. 



The recent experiments of Mr. Crookes, showing retardation 

 of the radiometer with extreme exhaustion, seem to indicate 

 that heat-rays, like the electric discharge, demand a certain 

 amount of atmospheric matter as their carrier. 



I cannot conclude these hasty and imperfect notes, written 

 merely with suggestive intent, without quoting a passage from 

 thft preface to the ' Correlation of Physical Forces,' which, 



