IX. The Bakerian Lecture. —On repulsion resulting from radiation. –Part V

Abstract

220. In this Part I propose to give the results of some researches which, during the past twelve months, have occupied much of my time. The researches include a quantitative examination of the repulsion exerted by a standard flame shining on pith and mica disks, coated with various powders, chemical precipitates, &c., and suspended in vacuo in a torsion apparatus. The character of the incident radiation has been varied by straining it through water, alum, or coloured media; the action of good and bad conductors of heat has been compared; and the influence which favourable presentation of the experimental surface, by curvature or obliquity, has upon its movement has been investigated, with the result of throwing much light on some of the debated problems in molecular physics to which, by general assent, the repulsion resulting from radiation is held to be due. In every step of this investigation, theory and observation have gone hand in hand, and at each point gained it has been my endeavour to permanently record such experimental proof in the convenient form of an instrument, so as to have it available for further examination. The reaction, along lines of greatest molecular pressure, between the experimental surface and the fixed case containing it, has been examined. Experimental proof has been obtained, not only of the existence of such a reaction, but of the direction in which it is chiefly exerted; and the apparatus devised during this inquiry, to put each step of the theory to an experimental test, has led to the construction of a modification of the radiometer named the otheoscope , in which the reacting surface is no longer the side of the glass case, but is specially made with a view of getting the greatest sensitiveness in the moving parts of the apparatus. Owing to the increased delicacy of the instruments now made, it has been possible to detect the existence of molecular pressure when radiation falls on a black surface, in air of normal density.