XIII. On repulsion resulting from radiation.—Parts III. & IV

125. In my previous papers on this subject the experiments described have had for their object the demonstration of the broad facts of repulsion resulting from radiation. In Part I., after satisfying myself that the action was not due to air-currents or electricity, I went rapidly over bodies of the most diverse chemical and physical characters, organic and inorganic, metallic and non-metallic, dense and light, in spheres, disks, and thin plates, endeavouring to find, from their behaviour when free to move in a vacuum, what conditions were necessary to obtain the strongest movement under the influence of radiation, and what were unnecessary. I ascertained that chemical constitution had little or nothing to do with the action. I said (par. 75) “the law appears to be that the force exerted is in proportion to the extent of surface exposed, rather than in proportion to the mass. Much surface and extreme lightness are the requisites in selecting materials for the beam, index, or gravitating mass; and when the masses have the same specific gravity and extent of surface, their position in respect to the source of heat determines the extent of movement. Thus a cylinder of pith is more sensitive when arranged for the heat to act on its side than on its end.” I tried many experiments on the circumstances governing the position of the neutral point during exhaustion, and I proved that, within experimental limits, the nearer the vacuum approached perfection the stronger was the movement due to radiation. In Part II. I described many improved forms of apparatus by which the movements due to radiation could be studied in a more complete manner and numerical results be obtained; the action of the various kinds of radiation, from the obscure heatrays emitted by copper at 100° C. to the blue and ultra-violet rays of the spectrum, was examined, the interference caused by passing the rays through various screens was shown, and the phenomena of the neutral point were further discussed. Experiments were described which satisfied me that the hypothesis of the movements being due to evaporation and condensation at the surface would not account for all the facts of the case; and ample proof was afforded that “to get the greatest delicacy in these apparatus there is required large surface with a minimum of weight,” an apparatus for the quantitative examination of this law being described.