1881.] 



of Radiant Heat upon Gaseous Matter. 



309 



With this arrangement the number of sounding gases and vapours 

 was rapidly increased. But I was soon made aware that the glass 

 lenses withdrew from the beam its most effectual rays. The silvered 

 mirrors employed in my previous researches were therefore invoked ; 

 and with them, acting sometimes singly and sometimes as conjugate 

 mirrors, the curious and striking results which I have now the honour 

 to submit to the Society were obtained. 



Sulphuric ether, formic ether, and acetic ether being placed in 

 bulbous flasks,* their vapours were soon diffused in the air above 

 the liquid. On placing these flasks, whose bottoms only were covered 

 by the liquid, behind the rotating disk, so that the intermittent beam 

 passed through the vapour, loud musical tones were in each case 

 obtained. These are known to be the most highly absorbent vapours 

 which my experiments revealed. Chloroform and bisulphide of carbon, 

 on the other hand, are known to be the least absorbent, the latter 

 standing near the head of diathermanous vapours. The sounds ex- 

 tracted from these two substances were usually weak and sometimes 

 barely audible, being more feeble with the bisulphide than with the 

 chloroform. With regard to the vapours of amylene, iodide of ethyl, 

 iodide of methyl and benzol, other things being equal, their power to 

 produce musical tones appeared to be accurately expressed by their 

 ability to absorb radiant heat. 



It is the vapour, and not the liquid, that is effective in producing 

 the sounds. Taking, for example, the bottles in which my volatile 

 substances are habitually kept, I permitted the intermittent beam to 

 impinge upon the liquid in each of them. ~No sound was in any case 

 produced, while the moment the vapour-laden space above an active 

 liquid was traversed by the beam, musical tones made themselves 

 audible. 



A rocksalt cell filled entirely with a volatile liquid, and subjected to 

 the intermittent beam, produced no sound. This cell was circular 

 and closed at the top. Once, while operating with a highly ather- 

 manous substance, a distinct musical note was heard. On examining 

 the cell, however, a small bubble was found at its top. The bubble 

 was less than a quarter of an inch in diameter, but still sufficient to 

 produce audible sounds. When the cell was completely filled, the 

 sounds disappeared. 



It is hardly necessary to state that the pitch of the note obtained in 

 each case is determined by the velocity of rotation. It is the same as 

 that produced by blowing against the rotating disk and allowing its 

 slits to act like the perforations of a syren. 



* I have employed flasks measuring from 8 inches to f ths of an inch in diameter. 

 The smallest flask, which had a stem with a bore of about fth of an inch in 

 diameter, yielded better effects than the largest. Flasks from 2 to 3 inches in 

 diameter yield good results. Ordinary test-tubes also answer well. 



