Nov. f, 1877] 



NATURE 



was taken by Profs. Tait and Dewar (July, 1875), who showed 

 how the increase, resulting from rarefaction, in the mean length 

 of the path of the gaseous molecules would favour the action, 

 but the explanation in the form which they gave to it required 

 that the rarefaction should be carried far enough to make the 

 mean length of path of a molecule of gas great as compared 

 with the dimensions of the inclosing vessel. It has, however, 

 been pointed out by Prof. Zoilner {Pogq. Ann., February, 1877), 

 and more recently by Mr. Tolver Preston {Phil. Mag., August, 

 1877), that, in ihe majority of cases, this condition is far 

 from being fulfilled. On the other hand, the residual-gas 

 theory of the action of the radiometer received very im- 

 portant experimental support from Dr. Schuster's beautiful 

 demonstration (February, 1876) that the force exerted on the 

 discs was correlative with an equal opposite force exerted 

 upon the glass envelope. The complete proof that the action 

 was due in some way to the presence of residual gas was furnished 

 by Mr. Crookes's own discovery (June, 1876) that it rapidly 

 diminishes when the exhaustion is carried beyond a certain point 

 depending on the nature of the gas. The outstanding defect in 

 the theory was removed by Mr. Johnstone Stoney, who {Phil. 

 Mag., March and April, 1876) showed that the observed pheno- 

 mena might arise at a degree of rarefaction at v;hich the mean 

 length ot path of a molecule was still much below the distance 

 from the discs to the envelope, it being sufficient that this distance 

 should not be too great to allow the warming of the discs to cause 

 a sensible increase in the velocity with which the molecules struck 

 the glass. Mr. Stoney's form of the theory answers to all the 

 facts of the case, so far as I am acquainted with them, and it has 

 been confirmed and illustrated by Mr. Crookes with a numerous 

 series of remarkably beautiful and ingenious experiments. 



My object in thus tracing the chief stages in the growth of the 

 accepted theoretical explanation of the radiometer has been to 

 point out that the quality of mind which led Mr. Crookes to 

 reject the various suggested explanations of the phenomena he 

 had observed, so long as they were only approximate and did not 

 account for all his facts, was merely a further exemplification of 

 the quality which led him to the original discovery. If he had 

 been content to disregard a seemingly trivial fact he would never 

 have made this discovery at all, and if he had disregarded slight 

 defects in the explanations that were offered he would have 

 missed some of its most important consequences. I think that 

 this also might have been suitably included among the "Lessons 

 of the Radiometer." G. Carey Foster 



University College, London, October 27 



Has Dr. Carpenter"allowed himself to become possessed by 

 a "dominant idea?" From his letter in NATtJRE (vol. xvi. 

 p. 544), I infer that he might have taken the trouble to reply to 

 my article in the Ju'y number of the Nineteenth Century, had 

 he not thought that my assertions " were well known in the 

 scientific world to be inconsistent with fact." 



Some remark?, however, made by Prof. G. Carey Foster at 

 the British Association seem to have forced upon Dr. Carpenter 

 the conviction that he may have underrated my character /or 

 veracity, and that the "scientific world," at all events, is not 

 unanimous in regarding my "assertions" as falsehoods. Dr. 

 Carpenter therefore seeks in your columns to justify the state- 

 ments contained in his article on " The Radiometer and its 

 Lessons," in the Nineteenth Century for April last. 



When Dr. Carpenter declares my "assertions (i) . . . (2) 

 ... (3) " to be false, I have a right to demand that Dr. 

 Carpenter give my identical words, and not his own interpre- 

 tation of my words — an interpretation which is "inconsistent 

 with fact." 



To show Dr. Carpenter's inaccuracies in small things as well as 

 great, I may point out that he does not even quote correctly 

 the title of my article in the Nineteenth Century. His carele.'s- 

 ness in more important matter.i is of deeper consequence. In 

 order to enforce one of his dominant ideas "yet more fully 

 and emphatically," he tells us thit he applied himself to a 

 "careful reperusal of" my papers " with the most earnest desire 

 to present a true history of the whole mquiry." A most laud- 

 able determination ! And where, will it be believed, did Dr. 

 Carpenter, a Fellow of the Royal Society, go for information ? 

 To the Philosophical Transactions, where my papers are printed 

 at full length ? No ! He only referred to the " Proceedings of 

 thd Royal Society," a record, as every one knows, that contains 

 brief, and therefore imperfect abstracts of what is published in 

 full in the Transactions. 



In his "justification" Dr. Carpenter quotes a passage from 

 a lecture I delivered in 1874, on The Repulsion Accompanying 

 Radiation, commencing, " my own impression is," &c. Had 

 Dr. Carpenter quoted the next paragraph, which is necessary 

 to a correct interpretation of the sentence he did quote, your 

 readers would have been enabled to judge how far I advanced 

 theories of my own. My words were these : " I do not wish to 

 insist upon any theory of my own. . . . The one I advance is, 

 to my mind, the most reasonable, and, as such, is useful as a 

 working hypothesis, if the mind must have a theory to rest upon. 

 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 either of the theories I have already dis- 

 cussea." My next paragraph concludes with the following quo- 

 tation from Sir Humphry Davy: — "When I consider the 

 variety of theories which may be formed on the slender founda- 

 tion of one or two facts, 1 am convinced that it is the business of 

 the true philosopher to avoid them altogether. It is more 

 laborious to accumulate facts than to reason concerning them ; 

 but one good experiment is of more value than the ingenuity of 

 a brain like Newton's." 



With regard to my having " theorised on the subject," I have 

 never denied having done so, although I have on five or six 

 occasions specially stated that " I wished to keep free from 

 theories," and " unfettered by the hasty adoption " of theories. 

 But I do deny that I ever stated that my results were definitely 

 explained by the direct mechanical action of light. Your readers 

 will understand that an experimental research is necessarily and 

 slowly progressive, and that the early provisional hypotheses 

 have to be modified, and perhaps altogether abandoned, in 

 deference to later observations. Until my experiments confirmed 

 the explanation given by Mr, Johnstone Stoney, I adopted no 

 definite theory, and I contend that a trained physicist would fail 

 to gather from my published p)apers that I desired my first 

 impressions to be regarded as final 



Dr. Carpenter again attributes to me the terms "anew force," 

 or a "new mode of force," as applied to the repul>ion accom- 

 panying radiation. Unless Dr. Carpenter can point these words 

 out in my published papers, he has no right to place them between 

 inverted commas. 



But the chief burden of Dr. Carpenter's song is that " Mr 

 Crookes has another side to his mind, which mak'^s Mr. Crookes 

 the spiritualist almost a different person from Mr. Crookes the 

 physicist." I fail to see how the investigation of certain pheno- 

 mena called spiritual can make a man a spiritualist, even if he 

 comes to the conclusion that some of the phenomena are not due 

 to fraud. My position in this matter was clearly stated some 

 years ago, and I ask your permission to quote the following 

 passages from an article I published in 1871 : — " I have desired 

 to examine the phenomena from a point of view as strictly 

 physical as their nature will permit. ... I wish to be considered 

 in the position of an electrician at Valentia examining, by means 

 of appropriate testing instruments, certain electrical currents and 

 pulsations passing through the Atlantic cable ; independently of 

 their causation, and ignoring whether these phenomena are pro- 

 duced by imperfections in the testing ins'ruments them: elves, 

 whether by earth currents or by faults in the insulation, or 

 whether they are produced by an intelligent operator at the other 

 end of the line." 



From this stand-point I have never deviated. Can Dr. 

 Carpenter say that his position and mine, in respect to the 

 investigation of the phenomena ascribed to spiritualism, are so 

 very different ? He asserts that he has shown beyond doubt that 

 it is all imposture. But I would ask if this was proved to his 

 satisfaction twenty years ago, why does he still waste valuable 

 time in interviews and sittings with so-called mediums ? If I am 

 to be censured for having devoted time to this subject, such 

 censure must be doubly applicable to a man who commenced the 

 investigation when I was a child, and who cannot let the subject 

 drop whenever a new " medium " comes in his way. Does he 

 regard the subject as h's o^n special preserve, and may his 

 demonstrations against other explorers in this domain of mystery 

 be looked upon as the conduct of a gamekeeper towards a 

 suspected poacher? 



To impress on the world that he has no " antmus," Dr. Car- 

 penter says he " cordially" and " personally congratulated " me. 

 His words bring vividly to my mind the conversation, of which, 

 by the by, he has omitted an important part. It was at the 

 annual dinner of the Fellows of the Royal Society on November 

 30, 1875, when the royal medal was awarded to me. Dr. Car- 

 penter accosted me with great apparent cord-ality, and said. 



