332 
PROFESSOR TYNDALL ON THE ACTION OF FREE MOLECULES ON 
supposed the vapour to undergo. In other words—as regards the absorption of 
radiant heat, the vapour would pass, without breach of continuity, through all its 
stages of condensation into the liquid form of matter. 
A general law of molecular physics is, I apprehend, here illustrated. 
§• 8- Rhythmic Absorption of Radiant Heat by Gases and Vapours. 
Conclusive as the foregoing experimental argument must appear, as regards the 
action of free molecules upon radiant heat, I am nevertheless glad to supplement it 
by another of a totally different character. On the 29th of November, 1880, I had 
the pleasure of witnessing, in the laboratory of the Royal Institution, the remarkable 
experiments of Mr. Graham Bell, wherein musical sounds were evoked by causing 
an intermittent beam of light to impinge upon thin discs 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 Mr. 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, suddenly 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. “When 
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 trans¬ 
parent to heat-rays they would for the most part go through this vapour unabsorbed, 
and produce no sonorous effect. I think Professor 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 Professor 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, 
