PHYSICS. 479 



Tyudall has delivered a lecture at the Royal Institution on the action 

 of free molecules on radiant heat, and its conversion thereby into sound, 

 in which he gave a resume of the discussion on the question of the ab- 

 sorptive action of aqueous vapor and then showed the apparently in- 

 controvertible proof by means of the photophone of the correctness of 

 his early experiments. {Nature, January, xxv, p. 232; Arm. Chim. Phys., 

 April, V, xxv, p. 177.) 



Dufour has confirmed the hypothesis that in the radiophone the 

 radiant energy acts on the layer of air in contact with the solid body 

 without expanding sensibly this body. A wire of blackened brass, 

 stretched on the mounting of a lens, pressed against the lens a thin glass, 

 which gave Newton's rings; and it was observed that no change took place 

 in the rings when the wire was subjected to an intermittent beam of light. 

 A very thin spiral of blackened zinc terminated in a small carbon cone 

 connected with the positive pole of a battery and rested on a plate of 

 carbon connected with the negative pole, a telephone being in circuit. 

 No sound was perceived when an intermittent beam was thrown upon 

 the spiral. The increased effect with increased absorption in a gas was 

 well shown by putting carbon dust in dry air; a strong sound was 

 heard when the tube was shaken. That short waves also produce the 

 result was shown by using a bulb as a receiver in which hydrogen and 

 chlorine were set free by electrolysis. {J. Phys., April, II, i, p. 196.) 



Kalischer has observed that by using simply a selenium cell in the 

 circuit of a telephone, and without battery, a sound is produced w^hen- 

 ever intermittent light falls on the selenium whose pitch corresponds t > 

 the rate of intermittence. The result appears to be due to light, since 

 it is not modified by alum or by water, and it is destroyed by colored 

 glass, except yellow. The cell should have a low resistance. ( Carl. Rep., 

 p. 563 ; J. Phys., April, II, i, p. 197.) 



4. Specific Heat. 



Strecker has determined the ratio of the specific heat at constant press- 

 ure to the specific heat at constant volume, of chlorine, bromine, and 

 iodine, all in the condition of gas, by comparing by Kundt's method the 

 wave lengths produced by the same sound in air and in the gas, at tem- 

 peratures between 20° and 390°. The constants do not vary sensibly 

 with the temperature, and approach closely the theoretic values, 1.323 

 for chlorine, 1.290 for bromine, and 1.300 for iodine. The experimental 



values are: 



Sp. lie<at. Sp. heat const. 



Velocity. Ratio. const, press. vol. ref. to air 



Chlorine 205.3°^ 1.323 0.1135 0.2139 



Bromine 135.0 1.293 0.0550 0.2358 



Iodine 107. 7 1. 294 0. 03489 0. 2350 



( Wied. Ann., V, xiii, p. 20 ; J. Phys., April, II, i, p. 187.) 

 Thoulet and Lagarde, desiring to obtain the specific heat of small frag- 



