Radiant Heat, and its Conversion thereby into Sound. 453 



different result. It was hoped by both of us that our dif- 

 ferences would be settled during this visit. With my closed 

 experimental tube I showed him the neutrality of dry air and 

 the activity of humid air; and while the latter was in the 

 tube I detached the rock-salt plates and placed them in his 

 hand. He closely inspected them, passed his dry handker- 

 chief over them, and frankly and emphatically pronounced 

 them perfectly dry. I then executed in his presence the 

 experiments with the open tube, and reproduced the results 

 which I had previously published. I subjected the method of 

 compensation to a severe test, and showed him how exact it 

 could be made. He frankly confessed his inability to find 

 any flaw in my experiments, and, save in one particular, made 

 no attempt to reconcile our differences. He accounted for 

 the neutrality of dry air observed by me by pointing to my 

 thermopile, between which and the experimental tube a space 

 of air intervened. He argued, and justly argued, that 

 though the calorific rays were permitted to enter the tube 

 from a vacuum, if the air intervening between tube and pile 

 could produce the effect which he ascribed to it, the heat 

 would be robbed of its absorbable rays before the dry air 

 entered the tube, the subsequent neutrality of dry air being a 

 matter of course. The logic was good ; but its basis I knew 

 to be more than doubtful ; and I therefore asked him whether 

 a layer ^o °f an mcn thick between pile and tube would 

 produce any sensible effect. His reply was an emphatic 

 negative. In subsequent experiments, therefore, the conical 

 reflector was removed from my pile, and placed within the 

 experimental tube, its narrow end being caused to abut 

 against the plate of rock-salt. The face of the pile was then 

 brought within less than -^ of an inch of the rock-salt plate ; 

 and in this way my former measurements, which had declared 

 the pure air of our atmosphere to be a practical vacuum to 

 radiant heat, were verified to the letter. 



The well-earned fame of Magnus as an experimenter, and 

 his personal friendliness to myself, rendered it specially in- 

 cumbent on me to deal respectfully with every one of his 

 suggestions. He once intimated to me that the absorption, 

 which I had supposed due to aqueous vapour, might be really 

 due to the smoke and dust suspended in London air. To 

 meet this I carried air myself from the Isle of Wight, had it 

 carried from Epsom Downs and other places, and found the 

 aqueous vapour diffused in such air to be from 60 to 70 times 

 more energetic than the air itself. London air, moreover, 

 was freed from its suspended matter, and tested when dry : it 

 proved neutral. The self-same air was then rendered humid : 



