RADIANT HEAT, AND ITS CONVERSION THEREBY INTO SOUND. 
293 
on the influence of molecular constitution on the phenomena of radiation and absorp¬ 
tion. 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 brought 
before the Ttoyal Society,* * * § and on the 10 th of June I was able, by illuminating the 
dial of a galvanometer and casting its image upon a screen, to demonstrate in the 
Boyal Institution 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.t 
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, amyline, 
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 obtained with a few of these substances; and I will here 
confine myself to the remark that were the measurements there recorded multiplied a 
hundred-fold, they would fall far short of the number actually executed in 1859. 
With a 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.^ 
Without 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 established with a clearness and an evidence unattainable by any other means 
then existent. § 
Notices of the investigation having appeared in many English and continental 
* Proceedings of Royal Society, vol. x., p. 37. 
t Proceedings of Royal Institution, vol. iii,, p. 155. 
| Philosophical Transactions, 1861, Vol. 151, pp. 6 and 7. 
§ With moderate total heats the method of compensation is extremely easy of application; but when 
the total radiation is very large, some discipline is required to keep the galvanometer needle steady in 
its most sensitive position. With due training, however, perfect mastery over this difficulty may he 
obtained. 
