61_b_. PERRIN’S EXPERIMENT. CATHODE RAYS CHARGED WITH NEGATIVE ELECTRICITY. CORRESPONDING POSITIVE CHARGES PROPAGATED IN THE REVERSE DIRECTION AND PRECIPITATED UPON THE CATHODE. _Comptes Rendus_, CXXI., No. 20, p. 1130; _The Elect._, Lon., Feb. 14, ’96, p. 523.—Jean Perrin’s object was to discover whether or not internal “Cathode rays were charged with negative electricity.” That they were had often been assumed by others, namely, Prof. J. J. Thomson, who considered cathode rays as due to negatively charged matter moving at high speed. § 61_b_. Again, Prof. Crookes, principally, and others, showed that they were possessed of mechanical properties and that they were deflected by a magnet. § 59. Perrin called attention to the above investigations and also alluded to the theoretical considerations of Goldstein, Hertz and Lenard, who favored the analogy of cathode rays to light—whose phenomena are well answered by the accepted theory concerning assumed etherial vibrations, which, in both cases, have rectilinear propagation, § 57, excite phosphorescence, § 54 and 55, and produce chemical action upon photographic plates. Great ingenuity was displayed, as might be expected, in the manner in which Jean Perrin proved the proposition named in the title of this section, at the Laboratory of the École Normale and also in M. Pallet’s Laboratory. First, therefore, let the elements of the discharge tube be thoroughly understood. As usual, the disk N is the cathode, referring to accompanying Fig. 1. A, B, C, D, is a metal cylinder having a small opening at the right hand end toward the cathode. Concentrically, is a similar cylinder, acting as an electrical screen and having a like opening similarly located as indicated. It corresponds to and plays the part of the Faraday cylinder, being connected to earth. The principle involved in this apparatus was based upon the laws of influence, which permitted him to ascertain the introduction of electric charges within a conducting envelope, and to measure such charges. During the discharge, the cathode rays were propagated from the cathode to and within the cylinder A, B, C, D, which immediately and invariably became charged with negative electricity. To prove that the charge was due to the cathode rays, he deflected them away from the opening in the protecting cylinder E, F, G, H. The cylinder was not under these circumstances charged, the rays being outside. He went further and made some quantitative analysis in a rough way to begin with. He related: “I may give an idea of the amount of the charges obtained when I state that with one of my tubes, at a pressure of .001 m. of mercury, and for a single interruption of the primary coil, the cylinder A, B, C, D, received sufficient electricity to bring a capacity of 600 C. G. S. units to a potential of 300 volts.” Upon the principle of the conservation of energy, he was induced, he said, to search for corresponding positive charges. “I believe I have found them