1881.] on the Conversion of Badiant Heat into Sound. 177 



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

 to submit to the Members 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 vapoui', 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 

 extracted from these two substances were usually weak and some- 

 times 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 them- 

 selves audible. 



A rock-salt 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 adiather- 

 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. 



Thus, as regards vapours, prevision has been justified by experi- 

 ment. I now turn to gases. A small flask, after having been heated 

 in the spirit-lamp so as to detach all moisture from its sides, was 

 carefully filled with dried air. Placed in the intermittent beam it 

 yielded a note so feeble as to be heard only with attention. Dry 

 oxygen and hydrogen behaved like dry air. This agrees with my 

 former experiments, which assigned a hardly sensible absorption to 

 these gases. When the dry air was displaced by carbonic acid, the 

 sound was far louder than that obtained from any of the elementary 

 gases. When the carbonic acid was displaced by nitrous oxide the 

 sound was much more forcible still, and when the nitrous oxide was 

 displaced by olefiant gas it gave birth to a musical note which, when 



N 2 



