On the Change from Radiant Heat and Light into Sound. 377 



interval, and, as before, we may neglect — &o. and higher 



Po 

 powers, we have, as the value of the capacity, 



<-¥{'-<?[-3}- 



Thus a small correction should be applied to the value -p — , 



depending on the rate of absorption during the interval for 

 which contact is maintained with the battery. An approxi- 

 mation to this quantity may be obtained by charging the con- 

 denser for some time with a battery of known electromotive 

 force, and then allowing it to discharge itself at small intervals 

 of time through the galvanometer. On the whole, however, the 

 results of measurements made, neglecting this correction, are 

 fairly satisfactory. 



The capacity of a paraffin condenser was determined by 

 several observers during the past term at the Cavendish Labo- 

 ratory. Their results differed by from § to 1 per cent. The 

 standard used was not in all cases the same ; and the measures 

 obtained by one observer, comparing this same condenser with 

 two different standards, differed by about -J per cent. It 

 seemed possible to determine within 10 ohms, when each of 

 the resistances B, B/ was about 5000 ohms, the value of B 

 for which the initial kick was zero. 



L. On a convenient Term expressing the Change from Ra- 

 diant Heat and Light into Sound. By Ernest H. Cook, 

 B.Sc, F.C.S.* 



WHILE making his experiments which resulted in the 

 production of the photophone, Prof. Graham Bell 

 made the discovery that, when rays of light fall upon certain 

 substances, the molecules of those substances are thereby 

 caused to vibrate in such a manner as to give rise to soundf . 

 The mutual convertibility of the undulations which produce 

 light and those which produce sound was thus clearly estab- 

 lished. Thinking these sounds to be produced by rapid 

 changes of temperature in the body impinged upon, Prof. 

 Tyndall made a series of experiments in order to determine 

 the different powers of bodies to absorb heat. Operating with 

 an intermittent beam from the electric lamp, and certain 



* Communicated by the Author. 



t Lecture at the Boston Meeting of the American Association for the 

 Advancement of Science, August 1880. 



Phil. Mag. S. 5. Vol. 11. No. 69. Mag 1881. 2E 



