ACTION OF RADIANT HEAT 41 



are in utter disaccord with those obtained by other experimenters, 

 who have ascribed a high absorption to air and none to aqueous vapor. 

 The action of aqueous vapor being thus revealed, the necessity of 

 thorouo-hly drying the flasks, when testing other substances, becomes 

 obvious. The following plan has been found effective : Each flask is 

 first heated in the flame of a spirit-lamp until every visible trace of 

 internal moisture has disappeared, and it is afterward raised to a tem- 

 perature of about 400 0. While the flask is still hot, a glass tube is 

 introduced into it, and air, freed from carbonic acid by caustic potash 

 and from aqueous vapor by sulphuric acid, is urged through the flask 

 until it is cool. Connected with the ear-tube, and exposed immediately 

 to the intermittent beam, the attention of the ear, if I may use the 

 term, is converged upon the flask. When the experiment is carefully 

 made, dry air proves as incompetent to produce sound as to absorb 

 radiant heat. 



In 1868 I determined the absorptions of a great number of liquids 

 w^hose vapors I did not examine. My experiments having amply 

 proved the parallelism of liquid and vaporous absorption, I held un- 

 doubtingly twelve years ago that the vapor of cyanide of ethyl and 

 of acetic acid would prove powerfully absorbent. This conclusion is 

 now easily tested. A small quantity of either of these substances, 

 placed in a bulb a cubic inch in volume, warmed and exposed to the 

 intermittent beam, emits a sound of extraordinary power. 



I also tried to extract sounds from perfumes, which I had proved 

 in 1861 to be absorbers of radiant heat. I limit myself here to the 

 vapors of patchouly and cassia, the former exercising a measured ab- 

 sorption of 30, and the latter an absorption of 109. Placed in dried 

 flasks, and slightly warmed, sounds were obtained from both these 

 substances, but the sound of cassia was much louder than that of 

 patchouly. 



Many years ago I had proved tetrachloride of carbon to be highly 

 diathermanous. Its sounding power is as feeble as its absorbent 

 power. 



In relation to colliery explosions, the deportment of marsh-gas was 

 of special interest. Professor Dewar was good enough to furnish me 

 with a pure sample of this gas. The sounds produced by it, when 

 exposed to the intermittent beam, were very powerful. Chloride of 

 methyl, a liquid which boils at the ordinary temperature of the air, was 

 poured into a small flask, and permitted to displace the air within it. 

 Exposed to the intermittent beam, its sound was similar in power to 

 that of marsh-gas. The specific gravity of marsh-gas being about 

 half that of air, it might be expected that the flask containing it, when 

 left open and erect, would soon get rid of its contents. This, however, 

 is not the case. After a considerable interval, the film of this gas 

 clinging to the interior surface of the flask was able to produce sounds 

 of great power. 



