NA TURE 



\_Nov. r, 1877 



from considerations which, for fear of misrepresentation, I must 

 give in Dr. Carpenter's own words : — 



" After pointing out that 'there is no real difference between 

 heat and light, all we can take account of [I presume he means 

 physically, not physiologically] being difference of wave-length,' 

 he [Mr. CrookesJ thus continues : ' Take, for instance, a ray of 

 definite refrangibility in the red. Falling on a thermometer it 

 shows the action of heat ; on a thermopile it produces an 

 electric current ; to the eye it appears as light and colour ; on, a 

 photographic plate it causes chemical action ; and on the sus- 

 pended pith i^rawj-^j w^/Zisw.' Now (i) this motion being else- 

 where spoken of as due to the impetus given by a ray of light, 

 (2) a set of experiments being made to determine the mechanical 

 values of the different colours of the spectrum, (3) an observation 

 being recoided on the weight of sunlight (without the least inti- 

 mation that he was ' speaking figuratively ' as Mr. Crookes says 

 that he did to his audience at the Royal Institution), (4) the term 

 light-mill htvng used by himself as a synonym for 'radiometer,' 

 and (5) no hint whatever being given of the dependence of the 

 result (as argued by Prof. Osborne Reynolds) on a ' heat-reaction' 

 through the residual vapour, I still hold myself fully justified in 

 attributing to Mr. Crookes the doctrine of the direct mechanical 

 action of light, " 



Taking these points in order and using Dr. Carpenter's 

 numbers for reference, I may observe as to (i) that this seems to 

 refer to Mr. Crookes's statement of an " impression " in a passage 

 already quoted; with regard to (2) that Mr. Crookes having 

 found that "every ray from the ultra-red to the ultra-violet " 

 produced a mechanical effect under the circumstances of his 

 experiments, it was very natural that he should hope to get some 

 clue as to the nature of the action by finding what rays produced 

 the greatest effect ; of Dr. Carpenter's arguments (3), (4), and 

 (5), it is difficult to speak with the seriousness befitting their 

 author's many valuable services to the cause of science, and the 

 "due consideration of . . . his and my lelative positions." To 

 conclude that Mr. Crookes must have held a particular theory 

 from the fact that, when he had constructed an apparatus which 

 spun round on exposure to light, he called it a "Light-mill ;" 

 from his having neglected to give warning that he was " speaking 

 figuratively " when he talked of "weighing a beam of sun-light," 

 or from his having given no hint that he had adopted a rival 

 theory, is certainly not to exemplify the " strict reasoning based 

 on exact observation " which Dr. Carpenter recommends in the 

 paragraph with which he concludes both his article and his letter 

 to this Journal. 



A few sentences before the passage I have quoted. Dr. 

 Carpenter refers to the "whole phraseology " of Mr. Crookes's 

 papers of January 5 and February 5, 1876, as indicating "that 

 he then considered [the motion of the radiometer] as directly due 

 to the impact of the waves upon the surface of tl e moving mass." 

 This again seems to me a very unsound conclusion. The effect 

 to the elucidation of which these papers were devoted was un- 

 questionably due to the incident radiation, but whether as a 

 primary or as a secondary effect, was still a matter for discussion. 

 In my opinion the phraseology used in them implies no more 

 than this : it indicates a relation of cause and effect, but, for the 

 most part, leaves the question as to how the latter follows from 

 the former, entirely untouched. If, however. Dr. Carpenter will 

 refer to § 195 of the paper of February 5, as it is printed in the 

 Fhil. Trans, for 1876, he will see that Mr. Crookes did not then 

 attribute the motion to direct impact of the rays upon the surface 

 of the moving body, but rather to an elevation of its temperature, 

 and a consequently increased radiation of heat from its surface. 

 At the same time he will see that this suggestion is put forward in 

 a tentative and entirely undogmatic way. 



Dr. Carpenter next undertakes to show that Mr. Crookes laid 

 claim to the discovery of a " new force " or a "new mode of 

 force," finding his proof of this in a passage included in the 

 quotation from his letter that I have given above. Commenting 

 on this passage in the Nineteenth Century (p. 248), he says : 

 "To the //^r^^ attributes of radiation universally recognised by 

 physicists, Mr. Crookes proposes (in the passage already cited) to 

 add a fourth, the power of producing an electric current in 

 a thermopile ; and a fifth, the power of producing mechanical 

 motion when acting on light bodies freely suspended in a 

 vacuum," Again, if Dr. Carpenter had consulted the Philo- 

 sophical Transactions for 1876 (p. 361), he might have done Mr. 

 Crookes more justice and might have given him credit for the 

 discovery of a sixth attribute of radiation — (Mr. Crookes there 

 mentions one more effect which the same ray can produce : 

 "concentrate it on the hand by a lens, it raises a blister accom- 



panied with pain "), — and, if he had read a few lines further, he 

 might have spared himself the trouble of explaining to Mr. 

 Crookes that the electric current of a thermopile is not directly 

 excited by the incident radiation, for he would have found that 

 this action, in common with the pain and the blister and the 

 motion of the mercury in a thermometer, is there spoken of as 

 being an effect of heat. I think it must be evident to any one 

 who will read this passage attentively with its context (either in 

 Proc. Roy. Soc. [February 10, 1876], from which apparently Dr. 

 Carpenter quotes, or in the Phil. Trans., loc. cit.), that it has 

 nothing at all to do with either one or more new forces, but that 

 the whole gist of it is to assert that, whatever may be the mode 

 in which radiation produces mechanical force, the result is to be 

 attributed to it as a whole and not to a particular constituent 

 assumed for the purpose. 



As though with the object of covering a retreat, Dr. Carpenter 

 says, near the end of his letter, that " Pi of. G. Carey Foster will 

 doubtless be able to pick out points of detail in my article, as to 

 which faults may be found by a severe critic." I may therefore 

 point out that I have so far carefully confined myself to what he 

 himself singles out as the "main issues" of the question between 

 us, and that, in my further remarks, I shall treat the matter from 

 a still more general point of view. 



In speaking (in my address at Plymouth) of the " tendency" 

 of Dr. Carpenter's article, I meant to indicate that I referred in 

 what I said about it to what seemed to me to be its general drift 

 and tone, rather than to any particular passage or passages. And 

 my judgment of the drift of the article was formed not only from 

 what I found in it, but also from what I did not find there. For 

 example, if Dr. Carpenter had thought as highly as I do of Mr. 

 Crookes's work he would almost inevitably have pointed out 

 more emphatically than he did the really astonishing number, 

 variety, and laboriousness of his experiments ; he would also, I 

 think, have pointed out that (with the important exception of 

 Dr. Schuster) scarcely one of the numerous investigators, who, 

 in consequence of his researches, have occupied themselves more 

 or less with the radiometer, had obtained any significant experi- 

 mental result which Mr. Crookes himself had not anticipated ; 

 and he would have shown that the discovery of the radiometer, 

 while affording a remarkable illustration of the importance ot 

 following up unexplained though apparently trivial phenomena, 

 illustrates no less forcibly the truth that scientific discoveries are 

 not chance revelations, coming now to one and now to another, 

 but that they are made only by those who have eyes to see a clue 

 when it is offered them, and patience and skill to follow where 

 it leads. 



Turning to what the article did contain, I think it is not incor- 

 rect to say that it tended to produce the impression that Mr. 

 Crookes, more or less obstinately, and on insuffici-^nt grounds, 

 rejected a satisfactory explanation of his results. I will therefore 

 try to state, as shortly as I can, what seems to me to be the true 

 state of the case in relation to this point. 



Prof. Reynolds (in his paper read before the Royal Society on 

 June 18, 1874) undoubtedly showed that a mechanical reaction, 

 such as might account for the results obtained by Mr. Crookes, 

 might arise when heat is communicated from a solid surface to a 

 vapour or gas, but he did not (then at least) show that in Mr. 

 Crookes's vacua there was enough residual gas to produce the 

 results he ascribed to it. M'. Crookes, without disputing the 

 possibility of the action pointed out by Prof. Reynolds, made 

 experiments from which he concluded that it was insufficient to 

 explain the movements he had observed. (I must here remark 

 that Mr. Crookes did not say, as Dr. Carpenter asserts that he 

 did, that the explanation offered by Prof. Reynolds was one 

 that " it is impossible to conceive." His words were : "It is 

 impossible to conceive that in these experiments sufficient 

 condensable gas or vapour was present to produce the effects 

 Prof. Osborne Reynolds ascribes to it. After the repeated 

 heating to redness at the highest attainable exhaustion, it is diffi" 

 cult to imagine that sufficient vapour cr gas should condense on 

 the movable index to be instantly driven off by a ray of light, or 

 even the warmth of the finger, with recoil enough to drive I ack- 

 wards a heavy piece of metal." — Phil. Trans., 1875, p. 547. But 

 although Prof. Reynolds is unquestionably entitled to the credit 

 of having originated the fundamental idea and worked out many 

 of the details of the explanation that seems now to be generally 

 adopted, his explanation not only rested on a somewhat slender 

 experimental basis, but was theoretically incomplete, and in par- 

 ticular it did not show clearly why so high a degree of rare- 

 faction should be needed for the production of the phenomena in 

 question. An important step towards supplying this deficiency 



