RADIOMETER, an instrument invented by prof. William Crookes of London to measure the comparative intensity of radiation in different colored or heated rays. It consists of a number of delicate arms (usually fou•), supported at their center of horizon tal motion, and above the center of gravity, upon a needle-point, so that they may revolve with the least resistance. Each arm carries at its extremity a very light disk of pith or of mica, blackened upon one side, the blackened surfaces all facing in the same relative direction. The invention was the result of researches on the atomic weight of the metal thallium, in which it was sought to eliminate the error caused by the decrease of weight of a body heated in air. He placed a balance in an exhausted vessel, but this not suc ceeding he made the following experiment. In an exhausted vessel he suspended a pith bar, while another pith ba,r was suspended in a vessel containing a natural quantity of air, and found that the bar in the exhausted vessel was repelled by a hot body, while that in the unexhausted vessel was attracted by it. An apparatus, the initial step in the construction of the radiometer, was devised to attain quantitative results. A j shaped tube of glass had suspended in it, by a delicate glass fiber, a small horizontal rodicarry ing upon each end a blackened disk of pith. The suspending fiber was attached above near the upper end of the vertical arm, and also to the middle of the horizontal bar, bal ancing ff. Thus, then: was formed a sort of Coulomb's torsion balance, though not for electrical measurement. To observe and to measure the motions of the horizontal rod,
it had placed at its middle a mirror for relleCting a beam of light on a distant graduated horizontal scale, like that employed in sir William Thomson's marine galvanometer. The slightest revolving motion of the horizontal bar would then cause the reflected ray of light to traverse a considerable portion of the graduated scale. A pure: spectrum, obtained by a system of lenses, etc., acting on a beam of light reflected by a heliostat, was employed in its different parts successively to act upon the blackened disks of the balance. The repulsion produced by the different portions of the spectrum, fur equal areas, was for each as follows, comparatively: ultra red, 100; extreme red, 85; red, 73; orange, 66; yellow, 57; green, 41; blue, 22; indigo, 8.5; violet, ti; ultra violet, 5. The repelling power diminished inversely as the square of the distance between the disk and the source of radiation, a result naturally to be expected. lie found also that a black ened disk was repelled more than five times as much as a plain one. Upon this hint he constructed his radiometer. The cause of the motions has been differently explained, some attributing it to the increased freedom of kinetic vibrations of the residual mole cules in the exhausted vessel, and which are more active near the blackened surface; but the question is not settled. Mr. Crookes's investigations will probably lead to other important discoveries in regard to radiation and radiant matter.