ANOTHER LESSON FROM THE RADIOMETER. 



303 



" Before this instrument can be used as a pho- 

 tometer or light-measurer, means must be taken to 

 cut off from it all those rays coming from the can- 

 dle or gas which are not actually luminous. A 

 reference to the spectrum diagram (Fig. 5) will 

 show that at each end of the colored rays there is 

 a large space inactive, as far as the eye is con- 

 cerned, but active in respect to the production of 

 motion— strongly so at the red end, less strong at 

 the violet end. Before the instrument can be used 

 to measure luminosity, these rays must be cut off. 

 We buy gas for the light that it gives, not for the 

 heat it evolves on burning, and it would therefore 

 never do to measure the heat and pay for it as 

 light." 



Dr. Carpenter either failed to remember this ex- 

 plicit statement, or overlooked it. 



In referring to the kinetic theory of gases as 

 explaining the movement of the radiometer, Dr. 

 Carpenter seems to imply that the question is 

 altogether settled. He might, however, have 

 found that this view is by no means universally 

 accepted. That the movement of the radiometer 

 is due not to any direct action of the solar (or 

 other) rays, but to their effect upon residual 

 gases, is now, indeed, owing to my more recent 

 researches, a matter of demonstration. That 

 some such explanation was in my mind at the 

 time of my first publication of the phenomena, 

 as one of the probable causes of the repulsion 

 resulting from radiation, is shown hy the follow- 

 ing quotations : 



" I object to the term perfect as applied to any 

 vacuum at present known. 



" That the residual gas in an air-pump vacuum 

 is capable of exerting considerable mechanical ac- 

 tion may be assumed by the phenomena attending 

 the passage of meteorites through the upper re- 

 gions of the atmosphere, their friction against the 

 air at an average height of sixty -five miles above 

 the earth's surface raising them to incandescence. 1 



" Whether the ethereal waves actually strike the 

 substance moved, or whether at that mysterious 

 boundary-surface separating solid from gaseous 

 matter, there are intermediary layers of condensed 

 gas which, taking up the blow, pass it on to the 

 layer beneath, are problems the solution of which 

 must be left to further research." 2 



My experiment on the movement of the glass 

 case of a radiometer 3 is termed " an ingenious ar- 

 rangement first devised by Dr. Schuster and sub- 

 sequently improved on by Mr. Crookes ; " where- 

 as, during the discussion which followed the read- 

 ing of Dr. Schuster's paper at the Royal Society 



1 " Philosophical Transactions," December 11, 1S73, vol. 

 elxiv., pp 507, 524. 



2 Philosophical Magazine, August, 1S74. 



3 " Proceedings ol the Royal Society," No. 168, 1S7G. 



on the 23d of March, 1876, I mentioned an ex- 

 periment which I had tried some time before, 

 bearing on his observations. I afterward tried 

 my own experiment in a modified form : and, as 

 the results were very decided, and appeared cal- 

 culated to throw light on many disputed points 

 in the theory of these obscure actions, I described 

 the experiment, and showed the apparatus at 

 work, at the next meeting of the Royal Society. 

 Without wishing in the least to detract from the 

 merits of Dr. Schuster, I may add that our two 

 experiments are entirely different as to mode of 

 arrangement and simplicity of exhibition. They 

 doubtless both prove the same thing — the exist- 

 ence of a reactionary force between the moving 

 fly and the glass case ; but, while Dr. Schuster's 

 experiment requires special arrangement of lime- 

 light, lantern, reflecting mirrors, torsion-threads, 

 etc., and then temporarily demonstrates only the 

 reactionary force, my experiment merely requires 

 a radiometer floating in a basin of water, and a 

 small magnet to fix the fly, when the case rotates 

 steadily and continuously. 



"While Dr. Carpenter was trying to prove from 

 my papers that I was committed to a wrong the- 

 ory which I was reluctant to abandon, how could 

 he avoid reading the following sentences V — 



" 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 theory on the slight- 

 est experimental basis. There is then great dan- 

 ger of their becoming advocates, and unconscious- 

 ly favoring facts which seem to prove their pre- 

 conceived 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 phe- 

 nomenon which is brought before it. Any theory 

 will account for some facts ; but only the true ex- 

 planation will satisfy all the conditions of the 

 problem, and this cannot be said of any theory 

 which has yet come to my mind. 1 



" As there is much discussion at present re- 

 specting the cause of these movements, and as some 

 misunderstanding seems to prevail as to my own 

 views on the theory of the repulsion resulting from 

 radiation, I wish to take this opportunity of re- 

 nioving the impression that I hold opinions which 

 are in antagonism to some strongly-urged explana- 

 tions of these actions. I have on five or six occa- 

 sions specially stated that I wish to keep free from 

 theories." - 



1 Quarterly Journal of Science, July, 1875. 



- " Proceedings ol the Royal Society," No. 1G8, 1S76. 



