Secondly, we require suitable heat vibrations, and I find that those given off in a room either warmed or chilled to sixty-three degrees are the very best possible; anything either much above this, or more especially, much below this, tending to weaken the results and to cheek the phenomena. Thirdly, we require suitable _musical_ vibrations, and, after carrying out a long series of experiments with musical instruments of all kinds, I find that the vibrations given off by the reed organ--termed "harmonium" or "American organ"--or by the concertina, are the most suitable, the peculiar quality of the vibrations given off by the reeds in these instruments proving to be the most suitable ones for use during the production of the phenomena; although on one or two occasions I have obtained good results without musical vibrations of any kind, but this is rare. Fourthly, we require the presence of a specially organized man or woman, termed _the sensitive_, one from whom it is alleged a portion of the matter used by the entity in the building up of its temporary body can be drawn, with but little chance of injury to their health. This point is one of vital importance, we are told, for it has been proved by means of a self-registering weighing-machine on which he was seated, and to which he was securely fastened with an electrical apparatus secretly hidden beneath the seat, which would at once ring a bell in an anteroom if he endeavored to rise from his seat during the experiment, that the actual loss in weight to the sensitive, when a fully materialized entity was standing in our midst, was no less than sixty-five pounds! Before employing any person, then, as a sensitive for these delicate, not to say dangerous, experiments, he or she should be medically examined, in the interests of both the investigator and the sensitive, and should their health prove to be in any way below par, they should not be permitted to take part in the experiment until their health is fully restored. I have been permitted to examine the sensitive at the moment when an entity, clad in a fully-formed temporary body, was walking amongst the experimenters; and the distorted features, the shrivelled-up limbs and contorted trunk of the sensitive at that moment proclaimed the danger connected with the production of this special form of phenomena far louder than any words of mine could do. Needless to say, sensitives for materializations are extremely rare, not more than two or three being found to-day amidst the teeming millions who inhabit the British Islands; although a few are to be found on the European continent, and several in North America, where the climatic conditions are said to be more favorable for the development of such persons. Now, what constitutes a sensitive, and why are they necessary?