1881.] Radiant Heat, and its Conversion thereby into Sound. 37 



action of the molecules on radiant heat is as clearly shown in these 

 experiments as in those previously conducted with the experimental 

 tube and thermopile. Upwards of eighty vapours have been tested 

 in regard to their sound-producing power. 



With regard to aqueous vapour, whose action upon radiant heat 

 even the latest publications on this subject describe as nil, it was 

 especially interesting to be able to question the vapour itself as to its 

 absorbent power, and to receive from it an answer which did not 

 admit of doubt. A number of bulbs about an inch in diameter were 

 placed under the receiver of an air-pump, with a vessel containing 

 sulphuric acid beside them. When thoroughly dry they were exposed 

 to an intermittent beam. The well-dried air within the bulbs proved 

 silent, while the slightest admixture of humid air sufficed to endow it 

 with sounding power. Placing a little water in a thin glass bulb, and 

 heating it nearly to its boiling point, the sounds produced by the 

 developed vapour are exceedingly loud. The bulbs employed in these 

 experiments are usually about a cubic inch in volume. They may, how- 

 ever, be reduced to one-fiftieth or even one one-hundredth of a cubic 

 inch. When a minute drop of water is vaporised within such little 

 bulbs, on their exposure to the intermittent beam loud musical sounds 

 are produced. 



It is to be borne in mind, that the heat employed in these experi- 

 ments, coming as it did from a highly luminous source, was absorbed 

 in a far smaller degree than would be the heat from bodies under the 

 temperature of incandescence. 



To render the correlation of sound-producing power and adiather- 

 mancy complete, all the gases and vapours which had been exposed to 

 the intermittent beam were examined as to the augmentation of their 

 elastic force through the absorption of radiant heat. A glass 

 cylinder, 4 inches long and 3 inches in diameter, had its ends 

 closed with transparent plates of rock-salt. Connected with this 

 cylinder was a narrow U"* u be, containing a coloured liquid which 

 stood at the same level in the two arms of the (J . The cylinder 

 could be exhausted at pleasure or filled with a gas or vapour. When 

 filled, the sudden removal of a double silvered screen permitted the 

 beam from the lime- light to pass through it, the augmentation of 

 elastic force being immediately declared by the depression of the 

 liquid in one of the arms of the (J -tube and its elevation in the other. 

 The difference of level in the two arms gave, in terms of water- 

 pressure, a measure of the heat absorbed. With the stronger vapours 

 it would be easy with this instrument to produce an augmentation of 

 elastic force corresponding to a water-pressure of a thousand milli- 

 metres. As might be expected, the intensity of the sounds corre- 

 sponded with the energy of the absorption, varying from " exceed- 

 ingly strong," "very strong," "strong," "moderate," "weak," to 



