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Sound by Radiant Energy. 519 



slipped over the mouth far enough down to prevent the 

 possibility of any light reaching the vapour above the surface. 

 Precautions were also taken to prevent reflection from the 

 bottom of the test-tube. An intermittent beam of sunlight 

 was then focused upon the liquid in the middle portion of the 

 test-tube by means of a lens of large diameter. 



Results. 



Clear water No sound audible. 



Water discoloured by ink ... Feeble sound. 



Mercury No sound heard. 



Sulphuric ether* Feeble but distinct sound. 



Ammonia 



Ammonio-sulphate of copper 



Writing-ink 



Indigo in sulphuric acid ... 

 Chloride of copper * 



The liquids distinguished by an asterisk gave the best 

 sounds. 



Acoustic vibrations are always much enfeebled in passing 

 from liquids to gases; and it is probable that a form of experi- 

 ment may be devised which will yield better results, by com- 

 municating the vibrations of the liquid to the ear through the 

 medium of a solid rod. 



Experiments with Gaseous Matter. 



On the 29th of November, 1880, I had the pleasure of 

 showing to Prof. Tyndall, in the laboratory of the Royal Insti- 

 tution, the experiments described in the letter to Mr. Tainter, 

 from which I have quoted above; and Prof. Tyndall at once 

 expressed the opinion that the sounds were due to rapid 

 changes of temperature in the body submitted to the action 

 of the beam. Finding that no experiments had been made 

 at that time to test the sonorous properties of different gases, 

 he suggested filling one test-tube with the vapour of sulphuric 

 ether (a good absorbent of heat), and another with the vapour 

 of bisulphide of carbon (a poor absorbent); and he predicted 

 that, if any sound was heard, it would be louder in the former 

 case than in the latter. 



The experiment was immediately made; and the result veri- 

 fied the prediction. 



Since the publication of the memoirs of Rontgen f and 

 Tyndall } we have repeated these experiments, and have ex- 



t Ann. der Phys. unci Chem. 1881, No. 1, p. loo. 

 \ Proc. Boy. Soc. yoI. xxxi. p. 307. 



