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nection with the Radiometer; moreover, the omissions and inaccuracies which occur in his historical review of my experiments and published researches on the subject would seem to deprive his inferences and conclusions of any value which they otherwise might have possessed.
We are told that when the theory of the Radiometer was under discussion at the Royal Society Professor Stokes confined himself to the statement that such mechanical action must lie outside the Undulatory Theory, which deals only with light as light--i.e. as producing visual phenomena.' The four last words are added by Dr. Carpenter to the observations of Professor Stokes. That the undulatory theory gives no account of the phenomena of light, save so far as they are connected with the vision of man and animals, is, to say the least, a startling revelation.
Again, we read that when the movement of the Radiometer was discussed it was noticed by several as anomalous, that the black should be the "driving" side of the disks, since it might have been anticipated that the mechanical action of light would manifest itself in pushing away the surface from which its rays are reflected, and that the surface into which they are absorbed would move towards the source from which the rays emanate.' Dr. Carpenter here omits the explanation of this apparent anomaly given by me at the Royal Society, and accepted as satisfactory by the eminent physicists present, to the effect that the rays falling on the white surface are simply reflected off without doing any work; but the rays falling on the black surface are absorbed, and their energy, disappearing in its original form, reappears as mechanical motion.
Dr. Carpenter next affirms 3 that I committed myself explicitly to the doctrine that the Radiometer is driven by light.' Later on I am accused of showing some lingering unwillingness to surrender this position; and I am then gravely censured for not knowing that heat causes the movement of the Radiometer. Now what are the facts? Let my own written words speak for me.
In 1873 I published the description of an experiment proving that every ray of the spectrum produced repulsion: the maximum action being in the extreme red. In March 1875, I wrote 5
Although I most frequently speak of repulsion by heat, it must be clearly understood that these results are not confined to the heating rays of the spectrum, but that any ray, from the ultra red to the ultra violet, will produce repulsion in a
So much for my earlier ideas on this subject. What are my later views regarding which, according to my critic, I still show 'some lingering unwillingness to surrender my position'?
1 P. 244.
$ P. 245.
Ibid. vol. clxv. p. 526.
In 1876 I wrote 6_
Is the effect due to heat or light? I cannot answer this question. The terms heat and light are not definite enough. The physicist has no test for light independent of heat. Light and colour are physiological accidents, due to the fact that a small portion near the middle of the spectrum happens to be capable of affecting the retina of the human eye. There is no real distinction between heat and light; all we can take account of is difference of wave-length.
After describing experiments with a pure solar spectrum, and giving numerical values for the motion-producing powers of the various coloured rays, I continue:
A comparison of these figures with those usually given in text-books to represent the distribution of heat in the spectrum will be a sufficient proof that the mechanical action of radiation is as much a function of the luminous rays as it is of the dark heat-rays.
Dr. Carpenter then accuses me of attributing the movement of the Radiometer to light. The very contrary is the case. I have always sought to guard against this misconception, insisting that every ray of the spectrum, visible or invisible, must cause motion. Hence I called the instrument the Radiometer-ray-measurer. Those who most persistently deny that light occasions the movements curiously enough continue to use the term 'light-mill.'
Dr. Carpenter introduces an account of an experiment I showed at the Royal Institution, on the evening of the 11th of February, 1876, with the words-This he called "weighing a beam of light."' Now, my actual words at the lecture were —
I want to ascertain the amount of pressure which radiation exerts on a blackened surface. I will put a ray of light on the pan of a balance, and give you its weight in grains. For I think in this Institution and before this audience I may be allowed a Scientific Use of the Imagination, and may speak of weighing that which is not affected by gravitation.
The italicised words render it evident that I was only speaking figuratively; and not, as Dr. Carpenter wishes to make it appear, that I conceived light to be a material substance.
Another misstatement follows on the next page, where Dr. Carpenter pronounces it as pure an assumption on Mr. Crookes's part to affirm that the mechanical action exerted by two flames of different kinds would measure their relative illuminating powers, as it would have been to say that their heating action would be proportional to their illuminating action, which we know perfectly well not to be the case,-the gas flame, as every one knows, having a much greater heating power than the candle flame, in proportion to the light it gives.'
Phil. Trans. vol. clxvi. pp. 360, 361, 362.
Proceedings of the Royal Institution, February 11, 1876, and Quarterly Journal of Science, April, 1876, p. 250.
Once again Dr. Carpenter omits part of my explanation. I will assume that he has read a portion of the description of the photometric experiment he criticises, given by me in the Proceedings of the Royal Society, No. 167, 1876. Why did he not read the next sentence, beginning?
By interposing screens of water or plates of alum, and so practically cutting off all the dark heat, the actual luminosity is measured.
Or perhaps he gained his information from my Royal Institution
Before this instrument can be used as a photometer or light measurer, means must be taken to cut off from it all those rays coming from the candle or gas which are not actually luminous. A reference to the spectrum diagram (fig. 5) will show that at each end of the coloured rays there is a large space inactive, as far as the eye is concerned, but active in respect to the production of motionstrongly 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 explicit 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 by the following 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 action, may be assumed by the phenomena attending the passage of meteorites through the upper regions of the atmosphere, their friction against the air at an average height of 65 miles above the earth's surface raising them to incandescence.9
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.10
8 Loc. cit.
9 Phil. Trans. December 11, 1873, vol. clxiv. pp. 507, 524.
10 Phil Mag. August, 1874.
My experiment on the movement of the glass case of a Radiometer 11 is termed an ingenious arrangement first devised by Dr. Schuster and subsequently improved on by Mr. Crookes;' whereas, during the discussion which followed the reading of Dr. Schuster's paper at the Royal Society on the 23rd of March, 1876, I mentioned an experiment which I had tried some time before, bearing on his observations. I afterwards tried my own experiment in a modified form; and as the results were very decided and appeared calculated 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 existence of a reactionary force between the moving fly and the glass case; but whilst Dr. Schuster's experiment requires special arrangement of lime light, lantern, reflecting mirrors, torsion threads, &c., 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.
Whilst Dr. Carpenter was trying to prove from my papers that I was committed to a wrong theory which I was reluctant to abandon, how could he avoid reading the following sentences?
Throughout the course of these investigations I have endeavoured 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 slightest experimental basis. There is then great danger of their becoming advocates, and unconsciously favouring facts which seem to prove their preconceived 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 phenomenon which is brought before it. 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 any theory which has yet come to my mind.12
As there is much discussion at present respecting 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 removing the impression that I hold opinions which are in antagonism to some strongly urged explanations of these actions. I have on five or six occasions specially stated that I wish to keep free from theories.13
Why, also, may I ask, has Dr. Carpenter when speaking of the Radiometer attributed to me the words 'new force' and 'new mode of force'? They are not my words. From which of my papers did he quote them?
11 Proceedings of the Royal Society, No. 168, 1876.
12 Quarterly Journal of Science, July, 1875.
Proceedings of the Royal Society, No. 168, 1876.
The time for a mere popular preliminary sketch of the Radiometer is gone by; that for its thorough and exhaustive appreciation has not yet come. Dr. Carpenter gives neither, but devotes the remaining part of his paper to exhibiting as a solitary lesson' the contrast assumed to exist between Mr. Crookes the physicist, investigating the phenomena of the Radiometer, and Mr. Crookes the 'spiritualist,' examining the manifestations of psychic force.' To use his own language, he brings into contrast with the admirable series of scientific investigations which led up to that invention, his [Mr. Crookes's] thoroughly unscientific course in relation to another doctrine of which he has put himself forward as the champion.'
In order to prove what he terms the duality' of my mental constitution, Dr. Carpenter contrasts my researches on the Radiometer with some experiments I made six years ago when I attempted to solve the mystery of the phenomena called spiritual, and he describes the apparatus I devised to test the alteration of the weight of suspended bodies in Mr. Home's presence, by mere contact and without pressure. In a lecture delivered at Chelsea, on the 19th of January, 1872, Dr. Carpenter referred to this experiment; and whether his description was accurate will be seen by an extract from a letter by Mr. A. R. Wallace dated February 15, 1872:
In the report of Dr. Carpenter's lecture at Chelsea there occurs a passage so extraordinary and so entirely misleading that I must beg you, in the interests of truth, to allow me to make a few remarks upon it. Dr. Carpenter is stated to have said that he would grapple with Mr. Crookes's Psychic Force;' and, in attempting to do so, exhibited an experiment intending to show (and which his audience must have believed really did show) that Mr. Crookes was ignorant of the merest rudiments of mechanics, and was deluded by an experiment, the fallacy of which an intelligent schoolboy could have pointed out. Dr. Carpenter, it is said, exhibited a glass of water poised against an equal weight upon a balance, and showed that by dipping a finger in the water—that is, by pressing with a force exactly equal to the weight of the water displaced by the immersed finger-you increased the weight on that side of the balance. Now, unless the audience were intended to believe that Mr. Crookes was ignorant of this childishly simple fact, and, further, that it completely accounted for the result of his experiment, for what purpose was this experiment shown? Yet if this is what it was intended to prove, then it becomes absolutely certain that Dr. Carpenter could never have read Mr. Crookes's account of his experiments given in October last in the Quarterly Journal of Science (for he would certainly not wilfully misrepresent the experiment), and was therefore in complete ignorance of what he was attempting to disprove. For, will it be believed, Mr. Crookes expressly states that 'dipping the hand to the fullest extent into the water does not produce the least appreciable action on the balance, the reason of which is sufficiently clear, for his woodcut shows, and his description tells us, that the vessel of water was not placed on the scale of a balance at all, but on a board exactly over its fulcrum or point of support at one end, while the distant end was suspended from a balance. Yet this balance showed a force of more than one pound exerted on it, when Mr. Home merely dipped the tips of the fingers of one hand in the water!
I have no wish in this article to discuss Mr. Home's psychic