THE RADIOMETER AND ITS LESSONS. 



31 



approaches a perfect vacuum. Now, while the 

 rate of rotation of the " mill " at first increases 

 with the degree of attenuation of the gaseous 

 atmosphere in which it moves, and ought, on Mr. 

 Crookes's original principle, to go on increasing, 

 it is found to attain its maximum at a certain de- 

 gree of exhaustion, and, when the exhaustion is 

 carried beyond that degree, to undergo a retar- 

 dation ; and this can scarcely be accounted for in 

 any other way than on the supposition that the 

 mechanical power exerted by the disturbance of 

 thermal equilibrium in the residual gas then di- 

 minishes at a more rapid rate than its mechani- 

 cal resistance to the rotation of the disks. Again, 

 it has been found that when the place of air in 

 several radiometers is taken by different gases 

 (as oxygen, hydrogen, carbonic acid, etc.), and 

 their globes are all exhausted to the same degree 

 as tested by a delicate pressure-gauge, their 

 " mills " rotate at different rates. Now, this is 

 exactly what would be expected on the kinetic 

 theory of gases ; since these different gases have 

 such diverse rates of molecular movement that 

 the reactionary forces generated by the disturb- 

 ance of thermal equilibrium will likewise vary ; 

 while, on the other hand, there seems no reason 

 whatever why the rate of rotation should be af- 

 fected by the nature of the residual gas (its elas- 

 tic force, and therefore the mechanical resistance 

 it exerts remaining the same), if the repulsion of 

 the disks is directly produced by radiant energy. 



Before adverting to the lessons which this re- 

 markable history seems to me to convey, I would 

 point out that this change of interpretation of 

 the facts discovered by Mr. Crookes does not in 

 the least diminish either the interest of the facts 

 themselves, or the merit of his discovery. Nor 

 is the value of his radiometer in any degree low- 

 ered by the demonstration that it does not (as 

 Mr. Crookes at first supposed) afford a mechani- 

 cal measure of radiant energy under any of its 

 aspects. What (according to present views) it 

 realiy does measure is the amount of " heat-reac- 

 tion " producible in gaseous atmospheres of dif- 

 ferent kinds and of different degrees of attenua- 

 tion. And such a precise method of measure- 

 ment appears more likely than any other mode 

 of investigation to furnish a test of that kinetic 

 theory of gases, the recent development of which 

 by Prof. Clerk-Maxwell is regarded by competent 

 judges as constituting (if it should receive such 

 verification) the most important advance ever 

 made in molecular physics. Most deservedly, 

 therefore, did Mr. Crookes receive from the Royal 



Society the award of one of its chief distinctions ; 

 and I would not be thought for one moment to 

 disparage his merits as the inventor of the radi- 

 ometer by now bringing into contrast with the 

 admirable series of scientific investigations which 

 led up to that invention what I cannot but regard 

 as his thoroughly unscientific course in relation 

 to another doctrine of which he has put himself 

 prominently forward as the champion. 



In the Quarterly Journal of Science for July, 

 1871, there appeared a paper by Mr. Crookes, en- 

 titled " An Experimental Investigation of a New 

 Force," in which he not only gave an account of 

 his own experiences with Mr. Home and other 

 spiritualistic " mediums," but indulged in very 

 unseemly reflections on the conduct of " scien- 

 tific men," whom he charges with having " re- 

 fused to institute a scientific investigation into 

 the existence and nature of facts asserted by 

 many competent and credible witnesses, which 

 they are freely invited to examine when and where 

 they please." The principal evidence adduced 

 by Mr. Crookes for the existence of this " new 

 force" was the power he attributed to Mr. Home 

 of being able to " alter the weight of bodies," the 

 chief proof of which was Mr. Home's depression 

 of a lever-board, whose farther end was attached 

 to a spring-balance, by laying upon it near its ful- 

 crum the tips of the fingers of both his hands ; 

 placing a small hand-bell under one hand, and a 

 little card match-box under the other, to satisfy 

 the by-standers that he was not himself exerting 

 any downward pressure. Now, " common-sense " 

 would teach that, if the end of a board kept up 

 by a spring goes down when a man's hands are 

 laid upon it, however near to its fulcrum, its go- 

 ing down is' due to the pressure of those hands ; 

 and the onus probandi obviously lies with those 

 who affirm that it is not so. Nothing would have 

 been easier than for Mr. Crookes, on the one 

 hand, to have carefully watched Mr. Home, to 

 have precisely imitated his whole procedure, and 

 to have done his best to depress the boaid to the 

 same degree by his own muscular effort ; and, on 

 the other hand, to have devised an "indicator" 

 for downward pressure (on the principle of Fara- 

 day's for lateral pressure), by which it could be 

 at once determined whether Mr. Home could de- 

 press the lever-board without such muscular ef- 

 fort. But, although Mr. Crookes, so far as I am 

 aware, has never published any proof obtained 

 from either of these test-experiments, although 

 explicitly challenged to do so, 1 he leaves on rec- 

 ord the claim to the possession of a power to 



1 Quarterly Review. October, 1871, p. 345. 



