312 Prof. Tyndall. Action of an Intermittent Beam [Jan. 13, 



dental observation will illustrate this point. A flask was filled with 

 coal-gas and held bottom upwards in the intermittent beam. The 

 sounds produced were of a force corresponding to the known absorp- 

 tive energy of coal-gas. The flask 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 could 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, formic ether, 

 sulphuric ether, or acetic ether, Jth of an inch in thickness, rendered 

 the transmitted beam powerless to produce any musical sound. These 

 liquids being transparent to light, the efficient rays which they inter- 

 cepted must have been those of obscure heat. 



A layer of bisulphide of carbon about ten times the thickness 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 

 sounds, the displacement was prompt and forcible. 



(Received January 10, 1881.) 



Further Experiments. 



Since the handing in of the foregoing note, on the 3rd of January, 

 the experiments have been pushed forward; augmented acquaintance 

 with the subject serving ouly to confirm my estimate of its interest 

 and importance. 



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

 extremely small quantities of fire-damp in mines. 



