five times as quickly as negative, under the action of an arc light, and that glass cuts the effect off while quartz transmits it. (For Elster and Geitel’s experiments, and those of Righi, _see_ Appendices, p. 115 _et seq._) This is rather a digression, but I have taken some pains to show it properly because of the interest betrayed by Lord Kelvin on this matter, and the caution which he felt about accepting the results of the Continental experimenters too hastily. It is probably a chemical phenomenon, and I am disposed to express it as a modification of the Volta contact effect[5] with illumination. [5] _See_ B. A. Report, 1884, pp. 502-519; or _Phil. Mag._, Vol. XIX., pp. 267-352. [Illustration: FIG. 9.--Zinc Rod discharging Negative Electricity in the very Ultra-violet Light of a Spectrum formed by a Quartz Train. The best discharging light is found far beyond the limits of the visible spectrum.] Return now to the Hertz vibrator, or Leyden jar with its coatings well separated, so that we can get into its electric as well as its magnetic field. Here is a great one giving waves 30 metres long, radiating while it lasts with an activity of 100 H.P., and making ten million complete electric vibrations per second. It is made of four large copper sheets soldered together two and two, strung up by thin rope to a gallery, and each pair connected with the other by several yards of No. 0 pure copper rod, interrupted by a pair of sparking knobs (Fig. 10). _Large Hertz Vibrator in action; Abel’s Fuse Detector; Vacuum Tube Detector; Striking of an Arc._ Its great radiating power damps it down very rapidly, so that it does not make above two or three swings; but nevertheless, each time it is excited, sparks can be drawn from most of the reasonably elongated conductors in the theatre of this Institution, and indeed from wire fencing and iron roofs outside this building. A suitably situated gas leak can be ignited by these induced sparks. An Abel’s fuse connecting the water pipes with the gas pipes will blow off; vacuum tubes connected to nothing will glow (this fact has been familiar to all who have worked with Hertz waves since 1889), electric leads, if anywhere near each other, as they are in some incandescent lamp-holders, may spark across to each other, thus striking an arc and blowing their fuses. This blowing of fuses by electric radiation frequently happened at Liverpool till the suspensions of the theatre lamps were altered. They had at first been held in position by wire guides, which served as collectors of the Hertz waves or impulses. [Illustration: FIG. 10.--Hertz Oscillator on reduced scale, ⅒th inch to a foot.] The striking of an arc by the little reverberating sparks between two lamp-carbons connected with the 100-volt mains I incidentally now demonstrate. An arc is started directly the large Hertz vibrator is excited at a distance.