Radiant Heat, and its Conversion thereby into Sound. 501 



Royal Institution, the remarkable experiments of Prof. Graham 

 Bell, wherein musical sounds were evoked by causing an in- 

 termittent beam of light to impinge upon thin disks of various 

 kinds of matter. I was soon convinced that the effects were 

 due to the rhythmic gain and loss of heat. Being occupied 

 with experiments on gases and vapours at the time, I thought 

 that they might be invoked to decide the nature of the action 

 revealed by Prof. Bell. The result was mentally clear before 

 the experiment was made. I pictured a highly absorbent 

 vapour, exposed to the shocks of the intermittent beam, sud- 

 denly expanding during the moment of exposure, and as 

 suddenly contracting when the beam was intercepted. Pulses 

 of an amplitude probably far greater than those obtainable 

 with solids would, I thought, be thus produced ; and these 

 pulses, if caused to succeed each other with sufficient rapidity, 

 would be sure to produce musical sounds. 



This idea was tested and verified on the spot. The ( Journal 

 of Telegraph Engineers' for December 8th, 1880, contains 

 the following record of what occurred : — " W hen Professor 

 Bell was good enough to show me his experiments, I happened 

 to be myself experimenting on the action of vapours upon 

 radiant heat. Old experiments had revealed, and new ones 

 had confirmed, the fact that, as regards the absorption of heat, 

 there existed vast differences between vapours. This is well 

 illustrated by the deportment of bisulphide of carbon and of 

 sulphuric ether, one of which is highly transparent, and the 

 other highly opaque to radiant heat. It occurred to me that, 

 if the action were due to the absorption of heat, we might 

 possibly extract musical sounds from sulphuric-ether vapour; 

 whereas bisulphide-of-carbon vapour being transparent to 

 heat -rays, they would for the most part go through this vapour 

 unabsorbed, and produce no sonorous effect. I think Prof. 

 Bell will bear me witness as to the result. We placed a 

 quantity of sulphuric-ether vapour in a test-tube, and allowed 

 an intermittent beam of light to strike upon the vapour far 

 above the liquid, and we heard distinctly a musical tone of a 

 pitch corresponding to the rapidity of the flashes. We then 

 took the bisulphide-of-carbon vapour, and tried it in a similar 

 manner, but neither Prof. Bell nor myself could hear any 

 trace of a musical sound" *. 



* Mr. Bell has given a perfectly accurate account of this occurrence in 

 the Philosophical" Magazine, vol. xi. p. 519. With reference to what 

 occurred on the 29th of November he writes thus : — " Professor Tyndall 

 at once expressed the opinion that the sounds were due to rapid changes 

 of temperature in the body submitted to the action of the beam. Finding 

 that no experiments had been made at that time to test the sonorous 

 properties of different gases, he suggested filling one test-tube with the 



Phil. Mag. S. 5. No. 84. Suppl. Vol. 13. 2 P 



