EADIANT HEAT, ANt> ITS CONVERSION THEREBY INTO SOUND. 
33 1 
intermittent beam, its temperature falls ; the quantity of vapour diminishes, and the 
sound sinks in intensity. On quitting the spirit flame, the bulb in some cases must 
have been near a red heat, but even at this temperature the vapour sounded loud. 
Eig. 7. 
It has, I think, been amply shown that when the molecules of a liquid are rendered 
free by vaporization they carry with them their absorbent power, liquids and vapours 
being pervious and impervious to the same quality of heat. Hence the inference, 
that prior transmission through a liquid of sufficient thickness ought so to sift a 
calorific beam as to render it powerless to act on the vapour of that liquid. Even with 
the loudest sounding vapours this proves to be the case, a layer of liquid -g-th of an 
inch thick being found generally sufficient to deprive the beam of its efficient rays, 
and the vapour of its sounding power. 
In transparent liquids, the visible rays have free transmission, the destruction of 
sounding power by such liquids must, therefore, be due to the absorption of the 
invisible calorific rays. This induction, which hardly needs verification, is nevertheless 
capable thereof. Many years ago I pointed out the astonishing transparency of dis¬ 
solved iodine to the invisible heat rays. Placed in the path of the intermittent beam, 
a layer of this substance, perfectly opaque to light, does not sensibly diminish the sound 
of transparent gases and vapours. To such substances, the iodine is exactly comple¬ 
mentary, arresting the rays which they transmit, transmitting the rays which they 
absorb, and, therefore, not interfering with the sounding power. 
That sounds may also be produced by the absorption of the visible rays is well 
exemplified by the deportment of iodine and bromine vapours, both of which yield 
with the lime light forcible sounds. Here the intervention of a transparent liquid, 
MDCCCLXXXII. 2 X 
