1881.] on the Conversion of Badiant Heat into Sound. 179 



After what has been stated regarding aqueous vapour we are 

 prepared for the fact that an exceedingly small percentage of any 

 hicrhly adiathermanons eras diffased in air suffices to exalt the sounds. 

 An accidental observation will illustrate this point. A flask was 

 filled with coal gas and held bottom upwards in the intermitteEt 

 beam. The sounds produced were of a force corresponding to the 

 known absorptive energy of coal-gas. The fliisk was then placed 

 upright, with its mouth open upon a table, and permitted to remain 

 there for nearly an hour. On being restored to the beam, the sounds 

 produced were far louder than those which cotdd be obtained from. 

 common air.* 



Transferring a small flask or a test tube from a cold place to the 

 intermittent beam it is sometimes found to be practically silent for a 

 moment, after which the sounds become distinctly audible. This I 

 take to be due to the vaporisation by the calorific beam of the thin 

 film of moisture adherent to the glass. 



My previous experiments having satisfied me of the generality of 

 the rule that volatile liquids and their vapours absorb the same rays, 

 I thought it probable that the introduction of a thin layer of its liquid, 

 even in the case of a most energetic vapour, would detach the effective 

 rays, and thus quench the sounds. The experiment was made 

 and the conclusion verified, A layer of water, liquid formic ether, 

 sulphuric ether, or acetic ether one-eighth of an inch in thickness 

 rendered the transmitted beam powerless to produce any musical 

 sound in the vapours. These liquids being transparent to light, the 

 efficient rays which they intercepted must have been those of obscure 

 heat. 



A layer of bisulphide of carbon about ten times the tTiiftVnftgg of 

 the transparent layers just referred to, and rendered opaque to light 

 by dissolved iodine, was interposed in the path of the intermittent 

 beam. It produced hardly any diminution of the sounds of the more 

 active vapours — a further proof that it is the invisible heat rays, to 

 which the solution of iodine is so eminently transparent, that are here 

 effectual. 



Converting one of the small flasks used in the foregoing experi- 

 ments into a thermometer bulb, and filling it with various gases in 

 succession, it was found that with those gases which yielded a feeble 

 sound, the displacement of a thermometric column associated with the 

 bulb was slow and feeble, while with those gases which yielded loud 

 soimds the displacement was prompt and forcible. 



On January 4 I chose for my sotirce of rays a powerful limp-light, 

 which, when sufficient care is taken to prevent the pitting of "the 

 cylinder, works with admirable steadiness and without any noise. I 

 also changed my mirror for one of shorter focus, which permitted a 



* The method here described is, I doubt not, applicable to the detectioa of 

 extremely small quantities of fire-damp in mines. 



