quently improved on by Mr. Crookes. A Radiometer, on whose arms a magnet is fixed, is floated in a vessel of water, round which four candles are fixed, so as to keep the mill in rotation. When a powerful magnet is brought near the outside of the globe, the arms immediately stop; but at the same time the globe begins to revolve slowly in the opposite direction, and continues to do so as long as the candles burn, and the arms remain fixed by the magnet. When the magnet is removed, the mill' begins to rotate in its original direction, and the glass envelope quickly comes to rest.

Now, as the existence of a reactionary force, which seems unmistakably indicated by this experiment, cannot be accounted for on the supposition that the mill' is driven round by the immediate mechanical impulse of radiation, whilst it is exactly what would be anticipated if the radiant energy acts calorifically on the residual gas, a very strong support is obviously afforded to the latter interpretation. And hence, although Physicists may differ as to the precise manner in which the unequal heating of the discs produces the movement, there is now, I believe, a very general accordance in the conclusion that this is the real modus operandi of the Radiant Energy; so that, instead of either a 'new force,' or a 'new mode of force,' we have simply a well-known mode of force acting under peculiar conditions.

This conclusion derives very striking confirmation from two of Mr. Crookes's more recent experiments, which seem to possess a crucial value. Having still further improved his 'Sprengel pump,' he has been able to carry the exhaustion of his globe to a yet greater degree than before, so that its internal condition more nearly 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 degree of exhaustion, and, when the exhaustion is carried beyond that degree, to undergo a retardation; 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 diminishes at a more rapid rate than its mechanical resistance to the rotation of the discs. Again, it has been found that when the place of air in several radiometers is taken by different gases (as oxygen, hydrogen, carbonic acid, &c.), 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 disturbance of thermal equilibrium will likewise vary; whilst, on the other hand,

* See Mr. Johnstone Stoney, in Philosophical Magazine, April 1876.

there seems no reason whatever why the rate of rotation should be affected by the nature of the residual gas (its elastic force, and therefore the mechanical resistance it exerts, remaining the same), if the repulsion of the discs is directly produced by Radiant Energy.

Before adverting to the lessons which this remarkable 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 lowered by the demonstration, that it does not (as Mr. Crookes at first supposed) afford a mechanical measure of Radiant Energy under any of its aspects. What (according to present views) it really does measure, is the amount of heat reaction' producible in gaseous atmospheres of different kinds and of different degrees of attenuation. And such a precise method of measurement 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 Professor ClerkMaxwell 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 Radiometer, 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, entitled '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 'scientific men,' whom he charges with having 'refused 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 fulcrum 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 bystanders 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

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when a man's hands are laid upon it, however near to its fulcrum, its going 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 board 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 Faraday's for lateral pressure), by which it could be at once determined whether Mr. Home could depress the lever-board without such muscular effort. But although Mr. Crookes, so far as I am aware, has never published any proof obtained from either of these testexperiments, although explicitly challenged to do so,3 he leaves on record the claim to the possession of a power to alter the weight of bodies which he originally advanced for Mr. Home, together with his own assertion of the existence of a 'new force,' and his charge against 'scientific men' for not experimentally investigating it. Their justification for abstaining from such an investigation was the utter unreliability of the evidence adduced, which consisted simply in Mr. Home's assertion that he was not exerting downward pressure, and in Mr. Crookes's belief that he could not thus have produced the effect; but having previously allowed himself to become possessed' by the spiritualistic idea, Mr. Crookes could not see this fallacy, accepted Mr. Home's assertion as a scientific fact, and scolded scientific men' for their incredulity! And yet, while asserting that they were 'freely invited to examine [these asserted facts] when and where they please,' Mr. Crookes admitted that Mr. Home's preternatural power could not be commanded, that he was subject to unaccountable ebbs and flows of this force,' and tha: 'it has but seldom happened that a result obtained on one occasion could be subsequently confirmed and tested with apparatus specially contrived for the purpose.'

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Now this is precisely what has happened over and over again within my own and others' experience of these pseudo-scientific phenomena, which depend upon the instrumentality of a Human personnel. Thus it was claimed by Mr. Lewis, a noted Mesmerist of twenty-five years ago, that he could not only draw his somnambules after him by mesmeric traction, but could raise them off the ground against the force of gravity.

When Mr. L. stood on a chair (says Dr. Gregory 4), and tried to draw Mr. H. without contact, from the ground, he gradually rose on tiptoe, making the most violent efforts to rise, till he was fixed by cataleptic rigidity. Mr. Lewis said that had he been still more elevated above Mr. H., he could have raised him from the floor without contact, and held him thus suspended for a short time, while some spectator should pass his hand under the feet. Although this was not done in my presence (continues Professor Gregory), yet the attraction upwards was so strong

3 Quarterly Review, October 1871, p. 345.

• Letters on Animal Magnetism, p. 351.

that I see no reason to doubt the statement made to me by Mr. Lewis and by others who saw it, that this experiment has been successfully performed.

Yet when a committee of Aberdeen Professors subsequently tested Mr. Lewis's powers, under conditions admitted by himself to be perfectly fair, not only did he entirely fail in his endeavour to control the actions of his subjects' from a distance, but, finding himself unable to keep either of them, when standing sideways against a wall on the foot nearest to it, in the erect position, he explained that he had never claimed any other power of overcoming the force of gravity, than that which he exerted in causing a subject lying on the ground, by the traction of his hand above him, to rise and stand upright. This is the Mr. Lewis whose pretensions have been recently endorsed by Mr. Alfred R. Wallace; a gentleman for whose achievements in the domain of natural science I have the same respect as I have for those of Mr. Crookes in the line of physical research, but all whose statements on this subject are vitiated (like those of Mr. Crookes) by his deficient knowledge of the abnormalities of human nature, by his want of due discrimination between facts and inferences, and by his disability to perceive how much greater should be the cogency of the evidence adduced to command our belief in statements of a most extraordinary kind, than that on which we rest our acceptance of the ordinary facts of daily life.

Thus, to revert to the cases just cited, the fact in the first of them was simply that the lever-board went down when Mr. Home's hands were laid upon it; and the testimony of Mr. Crookes and his friends was quite sufficient to justify others in accepting it as such. On the other hand, Mr. Crookes's assertion that the lever-board went down in obedience to some other force than that of Mr. Home's muscular pressure was not a fact, but an inference drawn by Mr. Crookes; and this inference he had no scientific right to draw, until he had assured himself by every conceivable test that Mr. Home did not and could not so depress it. So, again, the rising-up of Mr. Lewis's subject from the prostrate to the erect position, under Mr. Lewis's outstretched hand, was a fact as to which Professor Gregory's testimony may be unquestioningly accepted, since it involves no improbability whatever; but of Mr. Lewis's power to lift him off the ground and to keep him suspended in the air, we obviously require a much stronger assurance than the assertion made to Professor Gregory by Mr. Lewis and by others who saw it. And those who refused to accept that assertion at the time, were fully justified by Mr. Lewis's explicit disavowal of it to the Aberdeen professors a few months afterwards.

So, again, Mr. Wallace's recently reiterated affirmation of the possession of the clairvoyant power by Alexis, Adolphe, and other

5 Edinburgh Monthly Journal of Medicine, 1852.

somnambules, is merely the believer's inference from facts which no extraordinary testimony is needed to establish, viz. that they read books or played cards with their eyes bandaged, or deciphered words in closed boxes put into their hands. But the sceptic's 'common sense inference from the very same facts would be that, in the first case, the eyes of the supposed clairvoyants had not been effectually blinded; and, in the second, that they had either taken a sly peep into the boxes (as George Goble was detected in doing), or had guessed the word by fishing' with the help unconsciously given by the questioner, as I saw Alexis and Adolphe do many times. And that this latter inference is the true one, is indicated, on the one hand, by the failure of one performer after another under adequate test-conditions (as in the cases investigated by the French Academy of Medicine and Sir John Forbes, and in many besides), and, on the other, by the detection of the mode in which the cheat was practised. I am confident that Mr. Wallace cannot point to a single case of clairvoyance thoroughly investigated by a sceptical expert, which has survived. such investigation. But of cases which satisfied intelligent and truthful witnesses, upon whose testimony we should rely in the ordinary affairs of life, and who were yet afterwards proved to have been completely taken in, there are enough to show how little such testimony is worth as to matters requiring special qualifications for their thorough investigation.

Of the two distinct claims set up by Mr. Crookes, therefore, to the discovery of a new agency in nature, I hold the one to have been as scientific as the other was unscientific. The facts of radiant repulsion did not rest upon the unsupported testimony of Mr. Crookes and his friends; they could be exhibited to as many as wished to see them, and could be verified for himself by every one who could construct the apparatus. And while his inference from the first series of those facts (ascertained by the torsion-balance) was regarded by some of our most eminent Physicists as by no means improbable, there were few, if any, among those who saw the Radiometer spin round when a candle was brought near it, who did not for a time accept his view. In assuming, however, that there was such a quantitative relation between Radiant Repulsion and Light as justified the use of his Radiometer as a Photometer, Mr. Crookes undoubtedly went beyond what his facts warranted; and his claim to have weighed a beam of light' I feel sure that he would now abandon. But no sooner was adequate ground shown for calling in question his interpretation of the phenomena, and a vera causa found in an agency already known, than Mr. Crookes evinced the spirit of the true philosopher in varying his experiments in every conceivable mode, so as to test the validity of his original interpretation. And if he still shows some lingering unwillingness to surrender his position, it is no more than the best of us would probably feel under the like circumstances in regard to a pet hypothesis.

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