SOUND AND RADIANT HEAT. 653 



stronger than those obtainable from solid matter would probably be 

 thus produced, which, when rapid enough, would generate musical 

 sounds. The intensity of the sound would, of course, be determined 

 by the absorptive power of the gas or vapor. 



This idea was tested on the spot. Placing sulphuric ether in a test- 

 tube, and connecting the tube with the ear, the intermittent beam was 

 caused to fall upon the vapor above the liquid. A feeble musical 

 sound was distinctly heard. Formic ether was tried in the same way, 

 and with the same result. Bisulphide of carbon was then tried, but 

 the vapor of this liquid proved incompetent to generate a musical 

 sound. These results, which were in perfect accordance with those 

 previously enunciated by the lecturer, were first made public during a 

 discussion at the Society of Telegraph Engineers on the 8th of De- 

 cember, 1880. * 



It was obvious, however, that the arrangement of Mr. Bell a truly 

 beautiful one was not suited to bring out the maximum effect. He 

 had employed a series of lenses to concentrate his beam, and these, 

 however pure, would, in the case of transparent gases, absorb a large 

 portion of the rays most influential in producing the sound. The 

 lecturer, therefore, resorted to lenses of rock-salt and to concave mir- 

 rors silvered in front. He employed various sources of heat, including 

 that of the electric lamp. The lime-light he found very convenient. 

 With the lime-light and concave mirror, sounds of surprising intensity 

 were produced by all the highly absorbent gases and vapors. Among 

 gases chloride of methyl was loudest. Conveyed directly to the ear 

 by a tube of India-rubber, the sound of this gas seemed as loud as the 

 peal of an organ. Abandoning the ear-tube, and choosing a suitable 

 recipient for the gas, the sounds were heard at a distance of twenty 

 feet from their origin. As regards intensity, the order of the sounds in 

 gases corresponds exactly with the order of their absorptions of radiant 

 heat. 



Among vapors sulphuric ether stands highest, this result being in 

 part due to the great volatility of the liquid. But the intensity of the 

 sound is by no means wholly dependent on volatility. The specific 

 action of the molecules on radiant heat is as clearly shown in these ex- 

 periments as in those previously conducted with the experimental tube 

 and thermopile. Upwai'd of eighty vapors have been tested in regard 

 to their sound-producing power. 



With regard to aqueous vapor, whose action upon radiant heat 

 even the latest publications on this subject describe as nil, it was es- 

 pecially interesting to be able to question the vapor itself as to its ab- 

 sorbent power, and to receive from it an answer which did not admit 

 of doubt. A number of bulbs about an inch in diameter were placed 

 under the receiver of an air-pump, with a vessel containing sulphuric 

 acid beside them. When thoroughly dry they were exposed to an in- 



* See " Journal of Telegraph Engineers." 



