1 862.] on the Absorption and Radiation of Heai^ ^c, 405 



course would be a rhum^, first examines the deportment of chlorine 

 as compared with hydrochloric acid, and of bromine as compareu 

 with hydrobromic acid, and finds that the act of combination which 

 in each of these two cases notably diminishes the density of the gas 

 and renders the coloured gas perfectly transparent to light, renders it 

 more opaque for obscure heat. He also draws attention to the fact 

 that sulphur, which is partially opaque to light, is transparent to 54 

 per cent, of the rays issuing from a source of 100 C, while its com- 

 pound, heavy spar, which is sensibly transparent to light, is quite 

 opaque to the rays from a source of 100 C He demonstrates, in 

 confirmation of Melloni, the transparency of lampblack in thin layers; 

 but shows how irreconcilable its deportment to radiant heat is with 

 the idea generally prevalent at the present day, that lampblack ab- 

 sorbs heat of all kinds with the same intensity. 



All his experiments with gases have been repeated with a different 

 source of heat, and he finds the result still more pronounced than 

 formerly, that the compound gases far transcend the elementary ones 

 in absorptive power. Taking air as unity, ammonia, at 30 inches 

 tension, is 1195, this latter figure representing all the heat that issued 

 from the source. A layer of ammonia, 3 feet long, is perfectly black 

 to heat emanating from an obscure source. The coloured gases, 

 chlorine and bromine, though much superior in absorptive power to 

 the transparent elementary gases, are exceeded in this respect by 

 every compound gas that has been hitherto examined. When, instead 

 of tensions of 30 inches, we compare tensions of 1 inch, the differences 

 between the gases come out still more strikingly. At this tension, 

 for example, the absorption of sulphurous acid is eight thousand times 

 that of air. 



The speaker also referred to a new and extensive series of ex- 

 periments on tlie Absorption of Radiant Heat by Vapours. The least 

 energetic, as before, he finds to be bisulphide of carbon ; the most 

 energetic, boracic ether. He shows that the absorption of the latter 

 vapour (which is quite transparent) at O'l of an inch of tension 

 is 600 times the absorption of the densely coloured vapour of bromine, 

 while in all probability it is 186,000 times that of air. 



The speaker was led by a series of perplexing experiments, which 

 are fully described in a Memoir recently presented to the Royal 

 Society, to the solution of the following remarkable and at first sight 

 utterly paradoxical problem — " To determine the absorptio7i and 

 radiation of a gas or vapour without any source of heat external 

 to the gaseous body itself" 



When air enters a vacuum it is heated by the stoppage of its 

 motion ; when a vessel containing air is exhausted by an air-pump, 

 chilling is produced by the application of a portion of the heat of the air 

 to generate vis viva. Let us call the heating in the first case dynamic 

 heating, and the chilling in the second case dynamic chilling. Let us 

 further call the radiation of a gas which has been heated dynamically, 

 dynamic radiation, and the absorption of a gas which has been chilled 



