36 



Dr. Tyndall. Action of Free Molecules on [Xov. 24, 



various solid substances, and to produce musical sounds. Mr. Bell's 

 previous experiments upon selenium naturally led liim to conclude that 

 the effect was produced by the luminous rays of the spectrum. The 

 contemplation of these experiments produced in the lecturer the con- 

 viction that the results were due to the intermittent absorption of 

 radiant heat. He was experimenting on vapours at this time. Substi- 

 tuting in idea gaseous for solid matter, he clearly pictured the sudden 

 expansion of an absorbent gas or vapour at every stroke of the calorific 

 beam, and its contraction when the beam was intercepted. Pulses far 

 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 vapour. 



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 vapour 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 vapour 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 

 December, 1880 * 



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

 bea/utiful 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 

 mirrors silvered in front. He employed various sources of heat, in- 

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

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

 intensity were produced by all the highly absorbent gases and vapours. 

 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 20 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 vapours 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 



* "Journal of Telegraph Engineers," vol. 9, p. 382, 



