Delusions, the form of which has changed from time to time, whilst their essential nature has remained the same throughout; and that the condition which underlies them all is the subjection of the mind to a dominant idea. There is a constitutional tendency in many minds to be seized by some strange notion which takes entire possession of them; so that all the actions of the individual thus 'possessed' are results of its operation. This notion may be of a nature purely intellectual, or it may be one that strongly interests the feelings. It may be confined to a small group of individuals, or it may spread through vast multitudes. Such delusions are most tyrannous and most liable to spread, when connected with religious enthusiasm ; as we see in the flagellant and dancing manias of the Middle Ages; the supposed Demoniacal possession that afterwards became common in the nunneries of France and Germany; the ecstatic revelations of Catholic and Protestant visionaries ; the strange performances of the Convulsionnaires of St. Médard, which have been since almost paralleled at Methodist 'revivals' and camp-meetings; the preaching epidemic of Lutheran Sweden; and many other outbreaks of a nature more or less similar. But it is characteristic of some of the later forms of these epidemic delusions, that they have connected themselves rather with Science than with Religion. In fact, just as the performances of Eastern Magi took the strongest hold of the Roman mind, when its faith in its old religious beliefs was shaken to its foundations, so did Appendices, B, C.

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the grandiose pretensions of Mesmer,-who claimed the discovery of a new Force in Nature, as universal as Gravitation, and more mysterious in its effects than Electricity and Magnetism,-find the most ready welcome among the sceptical votaries of novelty who paved the way for the French Revolution. And this pseudo-scientific idea gave the general direction to the doctrines taught by Mesmer's successors; until in the supposedSpiritualistic' manifestations a recur. rence to the religious form has taken place, which may (I think) be mainly traced to the emotional longing for some assurance of the continued existence of departed friends, and hence of our own future existence, which the intellectual loosening of time-honoured beliefs as to the Immortality of the Soul has brought into doubt with many.

I must limit myself, however, to the later phase of this history; and shall endeavour to show you how completely the extravagant pretensions of Mesmerism and Odylism have been disproved by scientific investigation all that is genuine in their phenomena having been accounted for by well-ascertained Physiological principles; while the evidence of their higher marvels has invariably broken down, when submitted to the searching tests imposed by the trained 'experts' whom I maintain to be alone qualified to pronounce judgment upon such matters.

Nothing is more common than to hear it asserted that these are subjects which any person of ordinary intelligence can investigate for himself. But the Chemist and the Physicist would most assuredly

demur to any such assumption in regard to a chemical or physical enquiry; the Physiologist and Geologist would make the same protest against the judgment of unskilled persons in questions of physiology and geology. And a study of Mesmerism, Odylism, and Spiritualism extending over more than forty years, may be thought to justify me in contending that a knowledge of the physiology and pathology of the Human Organism-corporeal and mental-of the strange phenomena which are due to the Physical excitability of the Nervous System, of the yet stranger results, the possession of the Mind by dominant emotions or ideas, of its extraordinary tendency to self-deception in regard to matters in which the feelings are interested, of its liability to place undue confidence in persons having an interest in deceiving, and of the modes in which fallacies are best to be detected and frauds exposed, is an indispensable qualification both for the discrimination of the genuine from the false, and for the reduction of the genuine to its true shape and proportions.

I hold, further, not only that it is quite legitimate for the enquirer to enter upon this study with that 'prepossession' in favour of the ascertained and universally admitted Laws of Nature, which believers in Spiritualism make it a reproach against men of science that they entertain; but that experience proves that a prepossession in favour of some occult' agency is almost sure to lead the investigator to the too ready acceptance of evidence of its operation. I would be among the last to affirm that there is not 'much more

in heaven and earth than is dreamt of in our philosophy;' and would be as ready as anyone to welcome any addition to our real knowledge of the great Agencies of Nature. But my contention is that no new principle of action has any claim to scientific acceptance, save after an exhaustive enquiry as to the extent to which the phenomena can be accounted for, either certainly or probably, by agencies already known; an enquiry which only 'experts' in those departments of science which deal with such agencies are competent to carry out. The assumption of a new agency, and the interpretation of phenomena in accordance with it, is a method which has proved so deceptive as to be now universally abandoned by men of truly philosophical habits of thought; being only practised by such as surrender their common sense to a 'dominant idea,' and deem nothing incredible which accords with their 'prepossession.'

The recent history of Mr. Crooke's most admirable invention, the Radiometer, is pregnant with lessons on this point. When this was first exhibited to the admiring gaze of the large body of scientific men assembled at the soirée of the Royal Society, there was probably no one who was not ready to believe with its inventor that the driving round of its vanes was effected by the direct mechanical agency of that mode of Radiant Force which we call Light; and the eminent Physicists in whose judgment the greatest confidence was placed, seemed to have no doubt that this mechanical agency was something outside Optics properly so called, and was, in fact, if not a new Force

in Nature, a new modus operandi of a Force previously known under another form. There was here, then, a perfect readiness to admit a novelty which seemed so unmistakably demonstrated, though transcending all previous experience. But after some little time the question was raised whether the effect was not really due to an intermediate action of that mode of Radiant force which we call Heat, upon the attenuated vapour of which it was impossible entirely to get rid; and the result of a most careful and elaborate experimental enquiry, in which nature has been put to the question in every conceivable mode, has been to make it (I believe) almost if not quite certain that the first view was incorrect, and that Heat is the real moving power, acting under peculiar conditions, but in no new mode.

No examination of the phenomena of Spiritualism can give the least satisfaction to the mind trained in philosophical habits of thought, unless it shall have been, in its way, as searching and complete as this. And when scientific men are invited to dark séances, or are admitted only under the condition that they shall merely look on and not enquire too closely, they feel that the matter is one with which they are entirely precluded from dealing. When, again, having seen what appears to them to present the character of a very transparent conjuring trick, they ask for a repetition of it under test conditions admitted to be fair, their usual experience is that they wait in vain (for hours it may be) for such repetition, and are then told. that they have brought an 'atmosphere of incredulity' with them, which prevents the manifestation.--Now