PHYSICS. 



oi) 



ondj the displacement at its center would be five-millionths of a centi- 

 meter. Since he found sound audible whose amplitude was less than 

 half this value, he coucludes that at present there is no reason for dis- 

 carding the obvious explanation that the sounds in question are due to 

 the bending of the plates under unequal heating. {N'atnre^ January, 

 1881, xxiii, 274.) 



Jamieson has devised a simple form of selenium cell, made of a i)iece 

 of plate glass or a glass tube, an inch in diameter and 3 inches long, 

 upon which are wound two parallel strands of No. 25 wire. Vitreous 

 selenium is melted into the spaces between the wires, and then annealed 

 in the usual way. One of these cells had a resistance of 5,740 ohms in 

 the dark and 3,450 in the light. An annular cell, placed outside the 

 tube of a swinging flame, transmitted its note perfectly to the tele- 

 phone, and, by placing a flat cell before the gas-flame of a Koenig man- 

 ometric capsule, and talking into the tube on the outer sound of the 

 membrane, conversation could be carried on. {NaHtre, Februarj', 1881, 

 xxiii, 354.) 



S. P. Thomi^son has suggested to the London Physical Society the use 

 of a conical instead of a parabolic reflector for the photophone. From 

 Adams's law, that the change in the resistance of selenium is directly as 

 the square root of the illuminating power, he finds that the change in 

 resistance of a cell will vary proportionally to its linear dimensions; 

 hence, selenium cells should be as large as possible, and the light should 

 be distributed over them uniformly. His cell was constructed of a slate 

 cylinder with a double screw-thread wound with wire and filled ^vdth 

 selenium. {PJnl Mag., April 1881, V, xi, 286 ; Nature, February, 1881, 

 xxiii, 331.) 



Tomlinson has found that a stick of annealed selenium gave twice the 

 deflection when coated with shellac varnish that it did when in its nat- 

 ural state. [Nature, March, 1881, xxiii, 457.) 



Tyndall has presented to the Koyal Society a paper on the action of 

 an intermittent beam of radiant heat upon gaseous matter, giving the 

 results of the use of the photophonic method to test the absorptive ac- 

 tion of aqueous vapor for heat — a subject long in controversy. The ex- 

 perimeuts were made by couvergiug the intermittent beam to a focus 

 within a flask containing the vapor to be examined. Sulphuric ether, 

 formic ether, and acetic ether gave loud musical tones, while those from 

 chloroform and carbon disulphide were barely perceptible, corroborating 

 his previous experiments. The power of amylene, ethyl and methyl 

 iodides, and benzene vapors to produce musical tones appeared to be 

 accurately expressed by their ability to absorb radiant heat. Gases 

 gave the same result. Turning now to water, a small quantity was 

 heated in a flask to a point near boiling; in the intermittent beam it 

 gave a powerful musical sound, even when no haze was present. Cool- 

 ing to 10° 0. did not prevent the sound from being loud, and even ordi- 

 nary air cooled in a freezing mixture for a quarter of an hour gave 



