Radiant Heat, and its Conversion thereby into Sound. 441 



action on a first trial to be thirteen times that of the air in 

 which it was diffused. 



It is not uninstructive to compare this approach to the 

 problem with that of a very distinguished man — the late 

 Professor Magnus, of Berlin *. Subsequent to me, he sub- 

 jected the aqueous vapour of our atmosphere to an experi- 

 mental test ; but he made the experiment under the assured 

 conviction that his result would be negative. " It could," he 

 says, " be foreseen with certainty that the small amount of 

 aqueous vapour taken up by air at ordinary temperatures 

 could exert no influence on the transmission.'''' I think it 

 must be obvious that if Magnus had gone through the disci- 

 pline to which I had been subjected, he would not have used 

 this language. His mistake, however, was a natural one. In 

 fact during the earlier stages of the inquiry my mind was 

 exactly in the condition of his mind — I also thinking, until 

 practically instructed to the contrary, that the action of 

 aqueous vapour at ordinary temperatures must be immeasur- 

 ably small. It is well known that Magnus tested his foregone 

 conclusion, and found it verified ; while I, on the other hand, 

 as above stated, justified mine. 



The various gases which had been examined in the experi- 

 mental tube with regard to their powers of absorption, were 

 next tested as regards their powers of radiation. Columns of 

 the heated gases were allowed to ascend in free air, and to 

 radiate against the pile. In this simple way the radiative 

 power of " transparent elastic fluids " was for the first time 

 established. The order in which the gases ranged themselves, 

 in regard to radiation, was exactly the order of their ab- 

 sorptions. Here, as in other cases, I instructed myself by 

 observing how gases might be made to play the part of solids. 

 Permitting, for example, a film of one of the stronger gases 

 to glide over a heated surface of polished silver, the radiation 

 from that surface was augmented as it might have been by a 

 coating of isinglass or lampblack. 



A surmise has been mentioned regarding new allotropic 

 conditions, as occurring to me amid the perplexities of my 

 earlier experiments. In one instance, that of electrolytic 

 oxygen, the surmise proved correct. My first experiments 

 indicated that the modicum of ozone which went forward 

 with the oxygen exerted four times the absorption of the gas 

 in which it was diffused f . Subsequently, by changing the 

 apparatus, and taking pains to augment the quantity of ozone, 



* In many respects my generous and helpful friend ; but, in regard to 

 this question, my steadfast antagonist for many years, 

 t Philosophical Transactions, vol. cli. p. 8. ' 



