Prof. Minchin, whose long and patient work in connection with photo-electricity is now becoming known, and who has devised an instrument more sensitive to radiation than even Boys’ radiomicrometer, in that it responds to the radiation of a star while the radiomicrometer does not, found some years ago that some of his light-excitable cells lost their sensitiveness capriciously on tapping, and later he found that they frequently regained it again while Mr. Gregory’s Hertz-wave experiments were going on in the same room. These “impulsion cells,” as he terms them, are troublesome things for ordinary persons to make and work with--at least I have never presumed to try--but in Mr. Minchin’s hands they are surprisingly sensitive to electric waves.[11] The sensitiveness of selenium to light is known to everyone, and Mr. Shelford Bidwell has made experiments on the variations of conductivity exhibited by a mixture of sulphur and carbon. Nearly four years ago M. Edouard Branly found that a burnished coat of porphyrised copper spread on glass or ebonite, diminished its resistance enormously, from some millions to some hundreds of ohms, when it was exposed to the neighbourhood, even the distant neighbourhood, of Leyden jar or coil sparks. He likewise found that a tube of metallic filings behaved similarly, and that both recovered their original resistance on shaking or tapping. Mr. Croft exhibited this fact recently at the Physical Society. M. Branly also made pastes and solid rods of filings, in Canada balsam and in sulphur, and found them likewise sensitive.[12] [11] _Phil. Mag._, Vol. XXXI., p. 223. [12] E. Branly, _Comptes Rendus_, Vol. CXI., p. 785; and Vol. CXII., p. 90. With me the matter arose somewhat differently, as an outcome of the air gap detector employed with an electroscope by Boltzmann (Fig. 16). For I had observed in 1889 that two knobs sufficiently close together, far too close to stand any voltage such as an electroscope can show, could, when a spark passed between them, actually cohere; conducting an ordinary bell-ringing current if a single voltaic cell was in circuit; and, if there were no such cell, exhibiting an electromotive force of their own sufficient to disturb a low resistance galvanometer vigorously, and sometimes requiring a faintly perceptible amount of force to detach them. The experiment was described to the Institution of Electrical Engineers in 1890,[13] and Prof. Hughes said he had observed the same thing. [13] _Journal_ Institution of Electrical Engineers, 1890, Vol. XIX., pp. 352-4; or “Lightning Conductors and Lightning Guards,” pp. 382-4. [Illustration: FIG. 16A.--Receiver in Syntonic Jar Experiment, with Knob Coherer and Tapper-back (_cf._ Fig. 4).]