1881.] of Radiant Heat upon Gaseous Matter. 



311 



intermittent beam was sent through, the space above the liquid. A 

 loud musical note was immediately produced. By the proper applica- 

 tion of heat to a liquid the sounds may be always intensified. The 

 ordinary temperature, however, suffices in all the cases thus far 

 referred to. 



In this relation the vapour of water was that which interested me 

 most, and as I could not hope that at ordinary temperatures it existed 

 in sufficient amount to produce audible tones, I heated a small quantity 

 of water in a flask almost up to its boiling-point. Placed in the inter- 

 mittent beam, I heard — 1 avow with delight — a powerful musical 

 sound produced by the aqueous vapour. 



Small wreaths of haze, produced by the partial condensation of the 

 vapour in the upper and cooler air of the flask, were, however, visible 

 in this experiment ; and it was necessary to prove that this haze was 

 not the cause of the sound. The flask was, therefore, heated by a 

 spirit-flame beyond the temperature of boiling water. The closest 

 scrutiny by a condensed beam of light then revealed no trace of 

 cloudiness above the liquid. From the perfectly invisible vapour, 

 however, the musical sound issued, if anything, more forcible than 

 before. I placed the flask in cold water until its temperature was 

 reduced from about 90° to 10° C, fully expecting that the sound 

 would vanish at this temperature ; but, notwithstanding the tenuity of 

 the vapour, the sound extracted from it was not only distinct but loud. 



Three empty flasks, filled with ordinary air, were placed in a freezing 

 mixture for a quarter of an hour. On being rapidly transferred to the 

 intermittent beam, sounds much louder than those obtainable from dry 

 air were produced. 



Warming these flasks in the flame of a spirit-lamp until all visible 

 humidity had been removed, and afterwards urging dried air through 

 them, on being placed in the intermittent beam the sound in each case 

 was found to have fallen almost to silence. 



Sending, by means of a glass tube, a puff of breath from the lungs 

 into a dried flask, the power of emitting sound was immediately 

 restored. 



When, instead of breathing into a dry flask, the common air of the 

 laboratory was urged through it, the sounds became immediately 

 intensified. I was by no means prepared for the extraordinary delicacy 

 of this new method of testing the athermancy and diathermancy of 

 gases and vapours, and it cannot be otherwise than satisfactory to me 

 to find that particular vapour, whose alleged deportment towards 

 radiant heat has been so strenuously denied, aflirming thus audibly its 

 true character. 



After what has been stated regarding aqueous vapour, we are pre- 

 pared for the fact that an exceedingly small percentage of any highly 

 athermanous gas diffused in air suffices to exalt the sounds. An acci- 



vol. xxxi. 2 A. 



