61_b_. THOMSON’S EXPERIMENT. VELOCITY OF CATHODE RAYS. _The Elect._, Lon., Oct. 5, ’94, p. 762; _Phil. Mag._, ’94.—The object of the experiment of J. J. Thomson was to determine whether the velocity approached that of light or that of molecules. The apparatus he employed involved the rotating mirror, which was fully described in _Proc. Royal So._, ’90, slightly modified. The rays were caused to produce phosphorescence, while the mirror was so adjusted that when at rest, the two images on the phosphorescent strips appeared in the same rectilinear line. Many other elements comprised the apparatus. All the steps were performed carefully and according to the best methods, but the results are those which in this experiment are of particular interest, for by knowing the velocity of the rays, their nature is better appreciated and that of the X-rays can be better deduced. The velocity bore a close relation to that of the mean square of the molecules of gases at temperatures zero ° C. or in the case of hydrogen, 1.8 × 10^5 cm. per second. As compared with such a velocity, that of the cathode rays was found to be in the neighborhood of 100 times as great, and this agrees very nearly with the velocity of a negatively electrified atom of hydrogen acquired under the influence of the potential fall, which occurred at the cathode. In further evidence of the verity of this statement, he made a rough calculation upon the curve or displacement produced by a magnet upon the rays. § 59. He stated: “The action of a magnetic force in deflecting the rays shows, assuming that the deflection is due to the action of a magnet on a moving electrified body, that the velocity of the atom must be at least of the order we have found.” [Illustration: FIG. 1. ]