338 
PROFESSOR TYNDALL ON THE ACTION OF FREE MOLECULES ON 
however adiathermanous it may be, produces no sensible effect upon the sound, the 
reason being that it permits the particular rays which act npon the coloured vapours 
to pass freely through it. A layer of dissolved iodine, on the other hand, deprives the 
beam of its power of evoking sounds from either iodine or bromine vapour. 
The rotation which produces the maximum effect is soon ascertained by experiment. 
The sound is loudest when the pulses succeed each other in periods which invoke the 
resonance of the flask. I possess a hollow cone with a well-polished rocksalt base, by 
which this point is well illustrated. Tilling this cone with chloride of methyl, 
while the base is turned towards the source of heat, the apex of the cone being 
connected by a tube with the ear, sounds of extraordinary intensity are produced. 
Abandoning the ear-tube, the sound can be heard at a distance. But to obtain this 
effect the speed of rotation must be definite and constant. The maximum sound once 
obtained, either the lowering or the heightening of the speed rapidly enfeebles it. It 
is difficult by hand-turning to keep the rate of rotation constant. Hence the desir¬ 
ability of a mechanical arrangement, which would ensure the proper rapidity and 
necessary uniformity. One or two motive powers have been tried, including a small 
steam-engine and an electro-magnetic engine, but the arrangement has not yet been 
brought to perfection. 
§. 9. Manometric measurements. 
Some time before the visit of Mr. Graham Bell in November, 1880, I had inserted 
into my old experimental brass cylinder a narrow tube of glass, which being bent at a 
right angle a few inches above the cylinder could hold an index of coloured liquid in 
its horizontal portion. I had long known that the absorption of radiant heat must be 
accompanied by the expansion of the absorbing body, but thought that such expansion 
would furnish only a rough measure of the absorption. With ordinary sources of heat 
I found the expansion small, even when sulphuric ether occupied the experimental 
tube; but when a pair of stout carbons, rendered incandescent by a Siemens’ machine, 
were employed as a source, the liquid index was driven forcibly out of the narrow 
glass tube. 
The experimental tube, however, was but a rude manometer, and I therefore 
sketched and described to my assistant at the time, with a view to its construction, a 
handier instrument. The apparatus was to consist of a short tube with rocksalt ends, 
capable of being exhausted and filled with any required gas or vapour. Through this 
tube it was proposed to send a concentrated calorific beam, whose action on an absor¬ 
bent gas or vapour should be declared by the depression of a liquid column in one leg 
of a U tube, and its elevation in the other. Two rocksalt plates were to be employed 
with the view of allowing the beam free escape from the tube after it had done its 
work upon the gas or vapour. The warming of the apparatus by the reverberation of 
the heat would be thus avoided. The point aimed at was to effect the expansion of 
