Radiant Heat, and its Conversion thereby into Sound. 493 



Short experimental tube. 

 Pressure. Absorption. 



3-5 inches 22'5 per cent. 



7-0 „ 30-0 „ 



The agreement here is almost as close as that established 

 by the first experiment. 



With a still brighter source of heat the absorptions were : — 



Long tube. 

 Pressure. Absorption. 



1 inch 18*4 per cent. 



2 inches 25*7 „ 



Short tn.be. 

 3-5 inches ..... 18'8 per cent. 



7-0 inches 25-6 „ 



When, therefore, the density of the vapour varies inversely 

 as the length of it traversed by the calorific rays, the absorp- 

 tion remains constant. 



The hydride of amyl lends itself conveniently to experiments 

 of this character. With it, and with the lime-light as source, 

 the following measurements have been made : — 



Hydride of amyl. 

 Long experimental tube. 

 Pressure. Deflection. Absorption. 



1 inch 10° 12-8 



2 inches .... 15 19-2 



Total heat . 49'4 



Short experimental tube. 

 Pressure. Deflection. Absorption. 



3-5 inches. ... 10° 12-2 



7-0 „ ... . 15 18-3 



Total heat . 50'5 



The agreement here is close enough to illustrate the law, 

 the greatest difference being under 1 per cent. 



At this point the following entry appears in my note-book: — 

 "It might have been wise on my part to rest content with 

 the comparison of the long and short tubes with the lime- 

 light as source of heat. But for the sake of completeness I 

 wished to introduce the incandescent spiral. The fulfilment 

 of this wish — that is to say, the successful performance of a 

 single experiment — has cost me more than a week's work. 

 Once, however, committed to the task, I could not leave it 

 incomplete. 



