Radiant Heat, and its Conversion thereby into Sound. 437 



Df this inquiry. It, however, kept alive reflections on the 

 influence of molecular constitution on the phenomena of 

 radiation and absorption. Encountered continually by the 

 thought that in liquids and solids the pure molecular action 

 was, or might be, hampered by cohesion, the desire to bring, 

 if possible, free molecules under the dominion of experiment 

 beset me more and more. 



At the beginning of 1859 I definitely attacked this problem, 

 meeting at the outset difficulties and negations the reverse of 

 encouraging. But after some weeks of labour, I found 

 myself in secure possession of the result, that gases and 

 vapours exhibited, in relation to radiant heat, phenomena far 

 more surprising than those observed by Melloni in liquids and 

 solids. On the 26th of May, 1859, the subject was brouglit 

 before the Royal Society*; and on the 10th of June I was 

 able, by illuminating the dial of a galvanometer and casting 

 its image upon a screen, to demonstrate in the Royal Insti- 

 tution not only the fact of absorption, but the astonishing 

 differences of absorption which gases and vapours equally 

 transparent to light manifested in regard to radiant heat f. 



The following gases and vapours were then examined : — 

 Air, oxygen, hydrogen, nitrogen, carbonic oxide, carbonic 

 acid, nitrous oxide, coal-gas, ammonia, olefiant gas, bisulphide 

 of carbon, chloroform, benzol, iodide of ethyl, cyanide of 

 ethyl, formate of ethyl, acetate of ethyl, propionate of ethyl, 

 iodide of amyl, chloride of amyl, amylene, absolute alcohol, 

 amylic alcohol, methylic alcohol, ethylic ether, ethylamylic 

 ether, sulphuric ether, and some others. In the Philosophical 

 Magazine for 1862 I have given samples of the results ob- 

 tained with a few of these substances ; and I will here confine 

 myself to the remark that were the measurements there re- 

 corded multiplied a hundredfold, they would fall far short 

 of the number actually executed in 1859. 



With the view of compelling the feeblest gases and vapours 

 to show, if they possessed it, their capacity to absorb radiant 

 heat, the " method of compensation " was invented %. With- 

 out prejudice to the delicacy of the galvanometer, this method 

 enabled me to bring into play quantities of heat far greater 

 than those ever previously invoked, my object being so to 

 exalt the total radiation that a minute fraction of that total 

 should reveal itself to experiment. By this method not only 

 were the feebler gases and vapours coerced, but the vastness 

 of the diathermic range, if I may use the phrase, was esta- 



* Proceedings of Eoyal Society, vol. x. p. 37. 



t Proceedings of Eoyal Institution, vol. iii. p. 155. 



% Philosophical Transactions, 1861, vol. cli. pp. 6 & 7. 



