310 Prof. Tyndall. Action of an Intermittent Beam [Jan. 13, 



Thus, as regards vapours, prevision has been justified by experi- 

 ment. I now turn to gases. A small flask, after having been heated 

 in the spirit-lamp so as to detach all moisture from its sides, was care- 

 fully filled with dried air. Placed in the intermittent beam it yielded 

 a musical note, but so feeble as to be heard only with attention. Dry 

 oxygen and hydrogen behaved like dry air. This agrees with my 

 former experiments, which assigned a hardly sensible absorption to 

 these gases. When the dry air was displaced by carbonic acid, the 

 sound was far louder than that obtained from any of the elementary 

 gases. When the carbonic acid was displaced by nitrous oxide, the 

 sound was much more forcible still, and when the nitrous oxide was dis- 

 placed by olefiant gas, it gave birth to a musical note which, when the 

 beam was in good condition, and the bulb well chosen, seemed as loud 

 as that of an ordinary organ pipe.* We have here the exact order in 

 which my former experiments proved these gases to stand as absorbers 

 of radiant heat. The amount of the absorption and the intensity 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 distant 

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

 flame burnt tranquilly. A bubble of nitrous oxide (sp. gr. 1*527) 

 was then blown, and placed in front of the reed. The flame im- 

 mediately 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 

 experiment 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 (sp . 

 gr. O069) 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 



* "With conjugate mirrors the sounds with olefiant gas are readily obtained at a 

 distance of twenty yards from the lamp. I hope to be able to make a candle flame 

 effective in tbese experiments. 



