THE NEW MOTIVE FORCE.

Evening Star, Issue 3922, 18 September 1875, Page 3

THE NEW MOTIVE FORCE.

An Fn journal says :—" mid all the varied topics imperial, social, and general—which filled our columns the other day, it is possible that by far the most important, in its consequences, was enshrined in the report of the proceedings of (he R ya< Society at its weekly meeting. The reporc was brief, but described a sc entifio demonstration which may yet turn out to be one of the most wonderful, and far-reaching by which human knowledge has been enriched. The experimentalist on the occa>i n,i >. question was Mr William Orookes, IrR S., to whose genius and subtle skill in questioning .Nature we already owe the di.covtry of the metal thallium, a'ohg with a c >mplete establishment of its atomatic weight. For some time past; this accomplished physicist has been investigating thv- action of heat and "light upon bodies suspended in a vacuum ; and so far back as .i-. uguvfc, 1573, he communicated some curious f u:t« on the subject to the Royal society. Buo those which he exhibited upon the evening of the last meeting, to an a dience composed of many among the most distinguished of living philosopher*, of an entirely new character, and such as combined the greatest simplicity of apparatus with astonishing results It has always been thought, and I>t Balfour Stewart has even stated it as certain, that light, apart from heat, exercised no mechanical aotion ; so that the spectacle of luminous rays driving round and round a suspended bar or vane would have been called a miracle in times bygone. Yet this ia what Mr Creokes showed to his audience. He suspended a bar of pith upon a silken fibre in a bulb at the bottom of a tube from which the air had been exhausted* to complete rarefaction, and a lighted candle placed near the outside of the bulb forthwith set this bar turning round. By a second ingenious apparatus he showed h-*w the intensity of this action of light could he measured, a glass thread as fine as a spider's web being here used. But it will be justly objected that the candle radiates heat as well aslmht, and, meeting this, Mr Crookes exhi bited an arrangement wherein four dies of pith upon cross pieces of straw, delicately balanced on a point in the exhausted bulb, were subjected to light from whkh the heat had been shifted away by a screen of alum. When the ln>ht thu3 purified, fell on the blackened side of each disc, it drove it mote and more rapidly round, but had no such effect upon the white side« of the pith. So perfectly ean this mechanical force of light be measured, that the lecturer circulated a card containing the rates at which different mediums of light at different distances would drive his fairy mill. At 20 inches one candle takes 182 seconds to move the vane round, at five it accomplishes a revolution in eleven seconds ; but diffused daylight sends the little vanes spinning in less than two sreonda, and full sunshine drives them merrily along yet more quickly. 4 Green glass greatly diminishes the action, as also blue, while yellow and light-red glass retard it much less. The stronger the luminous source, the more vigorous iathe impelling power ; the electric and magnesium lights whirl the tiny vanes very rapidly, and thus this radiometer exhibited may be developed into an accurate measure of actinic force, affording us—to come out of fairyland into practical life—an excellent gauge for the illumina'iug power of our gas burners. "\ here was still, however, jono objection to meet bef ro the learned men had assembled could saftly allow that they saw before them light, pure light, set to work at the treadmi'l and exhibited as a motive power. Professor Osborne Reynolds, when these researches first became known to him, had suggested that the discs of pith went round because there was latent moisture in them, whi.h, being evaporated io the experiment, gave a resilient impulse to the littie apparatus. The lecturer exhibited a bulb, containing platinum vanes suspended on <\ plaiimim thread, which had b'-:en heated to leducsn while under continuous aud absolute exhaustion ; yet this, too, which could hardly contain any latent moisture, obeyed the in • visible push of the light rays as completely as the other contrivances. The opiuion expressed by Buch ' savans' as Psofeshor Stokes and Huxley, Sectors Hugginw and Carpenter, Messrs KormanLockyor, T)e la Rue, Siemens, and Gulton was that the demonstration of a great an<i amazing discovery stood pe-fect, and it'was geuen-liy agre/d that in the present' position of scint.fic knowledge the rehulta obtained were inexplicable by am known thoi-r : .