xxvn 



-chloroform. With regard to the vapours of aniline, iodide of ethyl, 

 iodide of methyl, and benzol, other things being equal, their power to 

 produce musical tones appeared to be accurately expressed by their 

 .ability to absorb radiant heat." The dry elementary gases, hydro- 

 gen, nitrogen, and oxygen, produced a musical note so feeble as to be 

 heard only with attention ; but when these dry gases were displaced 

 by carbonic anhydride, the sound was far louder than that obtained 

 from any of the elementary gases. Ammonia gas produced a loud 

 musical note. Now came the crucial test to be applied to aqueous 

 vapour. 



Obviously, if dry air and moist air produced practically the same 

 .slight effect in the intermittent beam, the conclusion of Magnus 

 would be right, whilst if the moist air produced a much louder sound, 

 the correctness of Tyndall's result would be clearly demonstrated. He 

 says : " In this relation the vapour of water was that which inter- 

 ested me most, and as I could not hope that at ordinary tempera- 

 ture it existed in sufficient amount to produce audible tones, I heated 

 a small quantity of water in a flask almost to its boiling point. 

 Placed in the intermittent beam, I heard I avow with delight a 

 powerful musical sound produced by the aqueous vapour. I placed 

 the flask in cold water until its temperature was reduced from about 

 90 to 10 C., fully expecting that the sound would vanish at this 

 temperature ; but, notwithstanding the tenuity of the vapour, the 

 sound extracted from it was not only distinct but loud. Three 

 empty flasks, filled with ordinary air, were placed in a freezing mix- 

 ture for a quarter of an hour. On being rapidly transferred to the 

 intermittent beam, sounds much louder than those obtainable from 

 dry air were produced." Thus was this controversy finally set at 

 rest. 



Interpolated between the magnetic and thermal investigations, or 

 carried on simultaneously with them, were his researches on the 

 physical properties of ice. These investigations were carried out 

 partly in the laboratory of the Royal Institution, but chiefly during 

 vacation rambles in Switzerland. They involved a vast amount of 

 laborious observation and acute reasoning, but in the light of recent 

 experiments, it would be rash to predict that the theory of fracture 

 and regelation, founded by Tyndall upon Faraday's original experi- 

 ments, will maintain its place as the true and only explanation of the 

 motion of glaciers. 



In connection with his glacier work stand Tyndall's frequent 

 ascents of Mont Blanc and other Swiss mountains. In one of these, 

 namely the ascent of Mont Blanc in August, 1859, the writer 

 accompanied him. The expedition was undertaken by Tyndall with 

 the especial object of establishing several self-registering thermo- 

 metric stations between Chamounix and the summit of the mountain. 



