Radiant Heat, and its Conversion thereby into Sound. 507 



what it contains ; for, were this the mode of heating, air 

 would be as sonorous as defiant gas. Nor are the pulses due 

 to the sudden vaporization of a liquid layer which might be 

 supposed to overspread the interior of the flask. When water 

 at a low temperature is purposely caused to cover the interior 

 surface, exposure to the beam produces sound of a certain 

 intensity. When the flask is so heated in a spirit-flame as to 

 chase away every trace of the adherent liquid, the exposure 

 of the pure vapour, then within the flask, to the beam, 

 generates a sound far louder than that produced when the 

 liquid film was there. Holding the bulb containing the hot 

 vapour for a little time in the intermittent beam, its tempera- 

 ture 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. 



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 per- 

 vious 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 \ 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 transmis- 

 sion; 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 dissolved iodine to the in- 

 visible 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 complementary, 

 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, 

 however adiathermanous it may be, produces no sensible effect 

 upon the sound, the reason being that it permits the particular 



