Radiant Heat } and its Conversion thereby into Sound. 505 



measured absorptions of a sufficient number of substances, in 

 relation to their sounding-power, shall be tabulated imme- 

 diately. 



The fact is worth a passing reference, that the chlorides of the 

 elements appear one and all to be feeble sound-producers, 

 because they are one and all feeble absorbers of radiant heat. 

 Many years ago I had found them highly diathermanous, and 

 accepted chloride of sodium as representative of the class. 

 Silicium chloride, for example, though very volatile, is weak as 

 a sound-producer. Tetrachloride of carbon and terchloride 

 of phosphorus are also volatile, but not strong. Bichloride of 

 tin, chloride of arsenic, chloride of titanium, and chloride of 

 sulphur are all feeble sound-producers. In these three cases 

 the boiling-points are high: but non-volatility is not the cause 

 of the weakness; for when the vapours are raised, by heating 

 their liquids, almost to the pressure of the atmosphere, they are 

 still but feebly sonorous. Whatever, then, the condition may 

 be which renders these substances pervious to radiant heat, it 

 appears to be a condition common to them all. 



In experiments with the chlorides care must be taken to 

 abolish all fumes. Bichloride of tin sounds loudly with fumes, 

 but is weak without them. Simply heating the upper part of 

 the flask frequently suffices to lower the sounds from loudness 

 almost to silence. 



The universality of its presence, and the discussions which 

 it has provoked, rendered the action of water- vapour especi- 

 ally interesting to me. I did not imagine at the outset that 

 the modicum of vapour diffused in atmospheric air at ordinary 

 temperatures could produce sonorous pulses of sensible in- 

 tensity. In my first experiment, therefore, I warmed water 

 in a flask nearly to its boiling-point. I heated the flask above 

 the water with the spirit-lamp flame, thus dissipating every 

 trace of haze, and then exposed the clear vapour to the inter- 

 mittent beam. The experiment was a virtual question put to 

 the vapour as to whether I was right or wrong in ascribing 

 to it the power of absorbing radiant heat. The vapour an- 

 swered by emitting a musical note which, when properly con- 

 verged upon the tympanum, seemed as loud as the peal of an 

 organ. When the temperature was lowered from 100° C. to 

 10° C, the sound did not vanish, as I expected it would. It 

 remained not only distinct but strong. The flasks employed 

 in these experiments were dried in a variety of ways, of which 

 I have already given some account, and which will suggest 

 themselves to every experimenter in this field. Taken open 

 from the laboratory, and exposed to the intermittent beam, 

 the flasks are always to some extent sonorous. Placed beside 



