PHYSICS, PROGRESS OF, IN 1894. 



657 



electro-motive force is developed proportionate 

 to the difference of temperature of the junctions. 

 The electro-motive force per 1 0. varied witli 

 concentration, the value for pure water being 

 about 8'6. For some salts it increases with con- 

 centration, while for others it diminishes. In all 

 cases the current goes from hot to cold through 

 the solution. Henri Bagard (" Annales de Chiinie 

 etde Physique," September) has investigated the 

 thermo-electric force between 2 electrolytes. A 

 small difference, which the author ascribes to 

 diffusion, was observed between measurements 

 taken with a heating and a cooling junction. 

 The electrolytes were separated by a membrane, 

 which was proved to have no effect except to 

 cause the electro-motive force to diminish slight- 

 ly with time. The Peltier and Thomson effects 

 were similar to those obtained with metals. Carl 

 Barus ("American Journal of Science," May) 

 considers that 2 metals are thermoelectrically 

 identical when the sign and the number of avail- 

 able molecular paths which the current (or, bet- 

 ter, the elementary charge) is free to take is the 

 same in both metals. C. C. Hutchins (" Ameri- 

 can Journal of Science," September) finds that 

 the best combination for a thermo-couple of anti- 

 mony and bismuth alloys is for one element bis- 

 muth with 2 to 5 per cent, antimony, and for the 

 other bismuth with 5 to 10 per cent. tin. 



Photoelectricity. Elster and Geitel give in 

 'Nature" (Sept. 6) a summary of their recent 

 photoelectric discoveries for the benefit of Eng- 

 lish readers. They find that clean plates of 

 aluminum, magnesium, and pure or amalga- 

 mated zinc are, when negatively electrified, dis- 

 charged in a few seconds by light from the sun 

 or a cloudless sky. The part of the spectrum 

 thus active extends from the blue upward. The 

 more electro-positive a metal is, the larger the 

 wave length capable of producing a photoelec- 

 tric discharge. Rubidium is sensitive even to 

 dull-red light. Polarized light has the greatest 

 effect when the plane of polarization is perpen- 

 dicular to that of incidence. The discharge al- 

 most entirely ceases in a strong magnetic field. 

 They find also that Hertzian waves pass through 

 a Geissler tube having an alkaline metal elec- 

 trode far more easily in daylight than in darkness. 

 In one case the disruptive discharge is stopped 

 by light, namely, when the sparks of an influ- 

 ence machine are allowed to pass between a brass 

 ball as anode and a clean zinc disk as cathode. 

 Lodge (British Association, 1894) finds that 

 when the inside of an electrified pewter pot 

 is illuminated it does not leak, but when the 

 edge is illuminated it leaks rapidly. Thus the 

 leakage appears to be a matter of surface tension 

 and not of potential. 



Phenomena of Exhausted Tubes. P. Lenard, 

 from his experiments on the passage of cathode 

 rays through a metallic window (" Annual Cyclo- 

 paedia," 1893, p. 623), and from more recent ex- 

 periments on very high vacua (Wiedemann's 

 " Annalen "), concludes that those rays are phe- 

 nomena of the ether, and not streams of electri- 

 fied molecules. Prof. Fitzgerald (London 

 "Electrician," March 25) disputes this conclu- 

 sion, and maintains that the fact that the rays 

 can be deflected by a magnet disproves it. 

 Though Lenard's vacua were very high, they 

 still contained 10 10 molecules to the cubic milli- 

 VOL. xxxiv. 42 A 



metre quite sufficient for a molecular stream. 

 Lenard (Wiedemann's "Annalen," May) thinks 

 that deflection does not invalidate his conclu- 

 sions, for if the rays are molecular, the deflec- 

 tion should depend on the pressure in the 

 medium through which they pass ; while he 

 finds that when they pass through an aluminum 

 partition from a chamber in which the pressure 

 is maintained to another in which it changes 

 the deflection remains constant. The change of 

 deflection observed when the pressure alters in 

 the first chamber is due to the production of a 

 different kind of rays. The rays that are most 

 easily diffused are most easily deflected. Gold- 

 stein (Berlin Physical Society, Feb. 2) distin- 

 guishes 5 kinds of cathode rays, all of which in- 

 tersect and are mixed in the cathodic light as 

 ordinarily observed : (1) Yellow rays of the first 

 zone, propagated in straight lines, non-phos- 

 phorescent, not affected by magnets. (2) Rays 

 of the second zone, which also move in straight 

 lines, but may be bent out of their course by a 

 magnet and phosphoresce when they strike the 

 inner wall of the tube. (3) Rays of the third 

 zone, propagated uniformly in* all directions, 

 able to turn corners, and throwing no shadows. 

 (4) Rays that produce inverted images of the 

 electrode and are stopped by screens. (5) Rays 

 that are ordinarily invisible, but phosphoresce 

 with the appearance of bright stars where they 

 fall on the wall of the tube. J. J. Thomson, to 

 test whether the cathode rays are streams of 

 molecules or ether phenomena, has measured 

 their velocity, and finds that it is not that of 

 light, but agrees very nearly with that which a 

 negatively electrified atom of hydrogen would 

 acquire under the influence of the fall of poten- 

 tial that occurs at the cathode. Sir David Salo- 

 mons (London Royal Society, June 21) finds that 

 bands are produced more easily in small tubes 

 than in large ones, and that they are difficult to 

 produce unless they reach to the glass of the 

 tube. An exceedingly minute current may pro- 

 duce bands which disappear on increasing it and 

 reappear when it is further increased. The dis- 

 appearance he believes to be due to an optical 

 illusion. After the first stage the apparent dark 

 bands probably indicate the position of the 

 bright bands, the apparent bright bands being 

 actually the overlapping parts of two bands. 

 Carl Kirn (Wiedemann's " Annalen," June) finds 

 that the spectrum of the afterglow of a Geissler 

 tube is almost precisely that of the first visible 

 light from a heated solid, namely, a band in the 

 yellow green which is seen as gray. The whole 

 afterglow phenomenon is exactly the reverse of 

 what is seen when a solid begins to glow. The 

 author regards the afterglow as the result of 

 chemical modification of the contents of the 

 tube. 



Dielectrics. Macfarlane and Pierce (" Phys- 

 ical Review," November-December, 1893, and 

 September-October, 1894) find that while thin 

 strata of solid or liquid dielectrics are equally 

 strong, whatever the thickness, thin strata of 

 gaseous dielectrics weaken as the thickness in- 

 creases. This seerns to be due to the ease with 

 which the gas allows discharge by convection; 

 for when a liquid layer is thick enough to permit 

 convection currents discharge takes place at a 

 lower difference of potential. Graetz and Fomm 



