546 



NA TORE 



{Oct. 25. 1877 



upon the surface oj the moving mass, and not secondarily through 

 the intervention of air-currents, electricity, or evaporation and 

 condensation." 



In a paper subsequently communicated to the Royal Society 

 (^Proceedings, March 12, 1875), Mr. Crookes characterised the 

 explanation of the "repulsion from radiation " offered by Prof. 

 Osborne Reynolds, as one which " it is impossible to conceive" the 

 phenomenon taking place in a c/iiv«/ca/ vacuum. At the same time 

 he stated that he was unprepared to offer any other explanation, 

 and that " he should avoid giving any theory on the subject 

 until a sufficient number of facts have been accumulated." 



After bringing out the Rodiometer,!however, he reverted (as it 

 seemed to me) to his previous " impression ; " the whole phrase- 

 ology of his papers of January 5 and February 5, 1876, appearing 

 at the time, not only to mystif, but to every one of the eminent 

 scientific friends with whom I conversed on the subject, to indicate 

 that he then considered the rotation as directly due to the impact 

 of the loaves upon the surface of the moving mass. Nor have I 

 ever imputed it to him as a matter of blame that he took this 

 view of it ; on the other hand I have stated over and over again 

 that this seemed the general impression of the distinguished 

 Physicists to whom we " outsiders " looked for guidance in the 

 matter. Anyone who remembers what took place at the Meeting 

 of the Royal Society at which Mr. Crookes's paper was read, 

 will, I feel sure, bear out this statement. 



I shall now specify more explicitly the grounds on which I 

 attributed to Mr. Crookes, no longer as an "impression," but as 

 a definite "interpretation " of his facts, that the rotation of the 

 Radiometer is due to the dii-ect impact of the waves, and chiefly (I 

 never said exclusively) to those of the luminous waves ; and 

 further attributed to him a claim to the discovery of a "new 

 force" or ** new mode of force." 



This key-note seems to me to be most distinctly struck in the 

 following passage : — After pointing out that "there is no real 

 difference between Heat and Light, all we can tike account of [I 

 presume he means physically, not physiologically] being difference 

 of wave-length," he 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 it causes motion." 



Now ( I ) this motion being elsewhere spoken of as due to the 

 imptttts given by a ray of light, (2) a set of experiments being 

 made to determine the tnechanical values of the different colours 

 of the spectrum, (3) an observation being recorded on the laeight 

 of sunlight (without the least intimation that he was " speaking 

 figuratively," as Mr. Crookes says that he did to his audience at 

 the Royal Institution), (4) the term Light-mill- being used by 

 himself as a synonym for " Radiometer," and (5) no hint what- 

 ever being given of the dependence of the result (as argued by 

 Prof. Osborne Reynolds) on a "heat-reaction" through the resi- 

 dual vapour, I still hold myself fuly justified in attributing to 

 Mr. Crookes the doctrine o{ the aired nuxhanical action of light ; 

 and I call on Prof. Carey Foster to prove — not that Mr. Crookes 

 himself did not hold that ductrine — but (which is a very different 

 thing) that I am not justified by Mr. Crookes's own language in 

 attributing it to him. 



That Mr. Crookes considered such action a "new force " or a 

 "new mode of force, " plainly appears from my previous citation ; 

 in which he ranks Motion as a mode of Radiant action additional 

 to Light, Heat, and Ac;inism, differing as much from either of 

 them as they differ from each other. If it does not mean this, 

 what does it mem ? 



So, if Mr. Crookes has not changed his mind as to the inter- 

 pretation of his facts, I ask (l) why he now repudiates as inap- 

 propriate the term \Light-mill adopted (if not originally given) 



^ Having never heard of any pliysical philosoplier from Seebeck to Sir 

 William Thomson, wlio fooked at tlie electric current generated in the 

 Thermopile as anytliing else tfian an efTect of the tteating (whether by con- 

 diiciioH or by radiation) q\ the two metats of which it was composed. 1 was 

 greAtfy surprised at findmg it ranked by IVIr. Crookes as one of the imiticdiate 

 modes of Radiant action ; and I cailed attention in^my " Radiometer " paper 

 to what I supposed to be his mistake on this point. It may be that in my 

 ignorance of the newest developments of thermo-electric theory (my know- 

 ledge of it not being later than 1872 , '" Everett's translation of Deschanel,"' 

 p. 652), I have here uninlenlionalfy " depreciated Mr. Crookes's merits;" and 

 I shall be quite ready to recant and apjfogise for my mistake, if Prof. G. C. 

 Foster wifl show that it is Dr. Carpenter, not Mr. Crcokes, who is liere 

 in the wrontj. 



2 It is impossible not to 5,ee, in the use of this term, a suggestion that the 

 vanes are driven round by the direct mechanic-il impetus of Light upon them, 

 in the same way as the sails of a Wind-mill are driven round by^the direct 

 impetus of the Wind. 



by himself? and (z) why does he now admit that dependence of 

 the movements upon the presence of residual gas, which he 

 originally affirmed to be impossible to conceive? 



I have carefully confined myself to the tnaln issues of this 

 question. Prof. G. Carey Foster will doubtless be able to pick 

 out points of detail in my article, as to which fault may be 

 found by a severe critic. But I venture to think that I have 

 said enough to prove that what I said on the subject was written 

 under the honest conviction that I had adequate ground for my 

 statements ; and that I shall at any rate be absolved from the 

 imputation of having ill-naturedly referred to the history of the 

 Radiometer for the purpose of putting Mr. Crookes in the wrong ; 

 the "lesson" with which 1 concluded the article being as 

 follows : — 



" The lesson which this curious contrast [the ' duality ' o ^ 

 Mr. Crookes's mental constitution, which I speak of as hiving 

 plenty of parallels in past times, to say nothing of the 

 present] seems to me most strongly to enforce, is that of the 

 importance of training and disciplining the whole mind during 

 the period of its development, ot cultivating scientific habits of 

 thought (by which I mean nothing more than strict reasoning 

 based on exact observation) in regard to n^ery subject, and of 

 not allowing ourselves to become ' possessed ' by any ideas or 

 class of ideas, that the common sense of educated mankind pro- 

 nounces to be irrational. I would not for a moment uphold that 

 test as an infallible one ; but it ought to be sufficiently regarded, 

 to make us question the conclusions which depend solely upon 

 our own or others' subjectivity, and to withhold us from affirming 

 the existence of new agencies in Nature, until she has been ques- 

 tioned in every conceivable way, and every other possibility has 

 been exhausted." [Op. cit., p. 256.) 



October 10 William B. Carpenter 



I HEAR from Dr. Carpenter that he is sending to you, for 

 publication m the next number of Nature, a statetnent intended 

 as a refutation of an opinion which I ventured to express, in my 

 address to the Mathematical and Physical Section of the British 

 Association at Plymouth, respecting an article on the "Radio- 

 meter," contributed by him to the Nineteenth Century for April 

 As Dr. Carpenter appears to have interpreted that expression of 

 opinion in a sense different from that which it was intended to 

 convey, I shall be much obliged if you will afford me space in 

 your next issue for a few words of explanation. 



The words which I used in referring to his article were these : 

 " An eminent and accomplished scientific man had published, 

 within the last few months, an account of the discovery of the 

 radiometer, the unmistakable tendency of which was, either 

 intentionally orunintentionally, to depreciate Mr. Crookes's merits, 

 and to make it appear that he had put a wrong interpretation 

 upon his own results." The word depreciate, which occurs here, 

 is, I am aware, susceptible of various shades of meaning, and 

 perhaps it would have been better if I had guarded myself 

 against the possibility of misconception that lurks in it. What 

 I meant was that Dr. Carpenter's account of Mr. (ilrookes's 

 researches was likely to make his readers form a lower estimate 

 of their scientific value than, in my opinion, they deserved ; 

 but whether or not it was intended to have this efiect I did not 

 undertake to say. I did not mean, and had not the smallest idea 

 of suggesting, that Dr. Carpenter had been guilty of intentional 

 and conscious unfairness towards Mr. Crookes. I should have 

 thought it entirely unnecessary to disavow this latter interpre- 

 tation of my words, and indeed should have considered it an 

 insult to Dr. Carpenter's reputation to suppose that anyone would 

 understand them in this sense, had he not himself (in a recent 

 correspondence with me) endeavoured to fix this meaning on 

 them in spite of my repeated assurances that it w.is not intended. 



I do not wish to say anything on this occasion in support of 

 the opinion which I have admitted that I did intend to express, 

 but I shall ask you to allow me to do so in a future number of 

 Nature. G. Carey Foster 



University College, London, October 14 



Mr. Wallace and Reichenbach's Odyle 

 As Mr. Wallace has attempted (presumably with Mr. 

 Crookes's editorial concurrence) to rehabilitate, in the July 

 number of the Quarterly Journal of Science, the Odyle-doctrine 

 of Baron Reichenbach, I think it well to slate that I yesterday 

 availed myself of an opportunity of personally asking my friend 



