Radiant Heat, and its Conversion thereby into Sound. 487 



Thickness of vapour stratum. Absorption. 



0*05 inch 2-1 per cent. 



0-1 „ 4-6 „ 



0-2 „ ...... 8.7 „ 



0-4 „ 14-3 „ 



0-8 „ 21-0 „ 



1-5 „ 34-6 „ 



2-0 „ 35-1 „ 



"We thus see that when the depth of the vapour stratum 

 augments from ^ of an inch to 2 inches, the absorption 

 augments from 2 per cent, to 35 per cent, of the total radia- 

 tion. 



It is only with sulphuric ether that an absorption of 2 per 

 cent, by the thinnest stratum could be obtained. With most 

 other vapours the action of a layer ^o °f an i ncn ^ u thick- 

 ness proved insensible. We may thus begin with a layer 

 which yields an absorption immeasurably small, gradually 

 augment the thickness of the absorbing layer until it becomes 

 a column 38 inches long, and find, throughout, the increase 

 of absorption running hand in hand with the increase of the 

 length of the absorbent layer. This result is utterly irrecon- 

 cilable with the hypothesis that liquid layers on the plates of 

 salt played any important part in my experiments. 



With regard to " vapour-hesion," I have to make the fol- 

 lowing further remarks. Let a thin plate of polished rock- 

 salt be placed upon, or against, the face of the thermopile, 

 with its lampblack removed, so as to expose a clean metallic 

 surface. Let an open glass tube end about a quarter of an 

 inch above the plate of salt; and through this tube let a 

 current of mixed air and vapour be gently urged downwards 

 against the plate. No fine spray, which might readily arise 

 from bubbling through a liquid, is to be permitted to mingle 

 with the vapour. My vapours, I may say, have been usually 

 formed, without bubbling, in large flasks, each containing a 

 portion of a volatile liquid, the vapour of which was per- 

 mitted to diffuse in the air of the flask. Fletcher's foot- 

 bellows were employed to urge the vapours forward. The 

 needle being at zero, or thereabouts, on causing the mixed 

 current to impinge upon the rock-salt, the needle immediately 

 swings aside, the deflection varying from 20° to 80° and more, 

 according to the delicacy of the galvanometer and the quality 

 of the vapour. 



From such experiments, which prove heat to be liberated 

 when vapour comes into contact with the salt, the condensa- 

 tion of the vapour has been inferred. I accept the inference, 

 202 



