ACTION OF RADIANT HEAT. 37 



in which my former experiments proved these gases to stand as ab- 

 sorbers of radiant heat. The amount of the absorption and the inten- 

 sity of the sound go hand in hand. 



A soap-bubble blown with nitrous oxide, or olefiant gas, and exposed 

 to the intermittent beam, produced no sound, no matter how its size 

 might be varied. The pulses obviously expended themselves upon 

 the flexible envelope, which transferred them to the air outside. 



But a film thus impressionable to impulses on its interior surface 

 must prove at least equally sensible to sonorous waves impinging on 

 it from without. Hence, I inferred, the eminent suitability of soap- 

 bubbles for sound-lenses. Placing a "sensitive flame" some feet dis- 

 tant from a small sounding reed, the pressure was so arranged that the 

 flame burned tranquilly. A bubble of nitrous oxide (specific gravity 

 1-527) was then blown, and placed in front of the reed. The flame 

 immediately fell and roared, and continued agitated as long as the lens 

 remained in position. A pendulous motion could be imparted to the 

 bubble, so as to cause it to pass to and fro in front of the reed. The 

 flame responded, by alternately roaring and becoming tranquil, to 

 every swing of the bubble. Nitrous oxide is far better for this experi- 

 ment than carbonic acid, which speedily ruins its envelope. 



The pressure was altered so as to throw the flame, when the reed 

 sounded, into violent agitation. A bubble blown with hydrogen 

 (specific gravity 0-069) being placed in front of the reed, the flame 

 was immediately stilled. The ear answers instead of the flame. 



In 1859 I proved gaseous ammonia to be extremely impervious to 

 radiant heat. My interest in its deportment when subjected to this 

 novel test was therefore great. Placing a small quantity of liquid 

 ammonia in one of the flasks, and warming the liquid slightly, the 

 intermittent beam was sent through the space above the liquid. A 

 loud musical note was immediately produced. By the proper applica- 

 tion of heat to a liquid the sounds may be always intensified. The 

 ordinary temperature, however, suffices in all the cases thus far re- 

 ferred to. In this relation the vapor of water was that which inter- 

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

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

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

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

 powerful musical sound produced by the aqueous vapor. 



Small wreaths of haze, produced by the partial condensation of the 

 vapor in the upper and cooler air of the flask, were, however, visible 

 in this experiment ; and it was necessary to prove that this haze was 

 not the cause of the sound. The flask was, therefore, heated by a 

 spirit-flame beyond the temperature of boiling water. The closest 

 scrutiny by a condensed beam of light then revealed no trace of cloudi- 

 ness above the liquid. From the perfectly invisible vapor, however, 

 the musical sound issued, if anything, more forcible than before. I 



