10 MM. Lecher and Pern tor on the Absorption 



return to this subject iu speaking of the absorption of aqueous 

 vapour. 



Description of the Apparatus employed and of the Methods 

 of Research. 



Guided by the considerations already given, and taught by 

 many failures, we finally arranged method and apparatus as 

 follows : — It was necessary to do away with rock-salt plates 

 and all reflection, and yet to have the thermopile as well as the 

 radia ting-surface which served as source of heat in the same 

 experimental space, so that there should be no conduction and 

 no currents of air. We believe, then, we have solved this 

 problem by means of a piece of apparatus which is represented 

 in the accompanying plate on a scale about one tenth of the 

 actual size. 



A is the special experimental vessel, an inverted glass bell-jar 

 with the open end upwards. The thermopile S is placed on 

 the bottom of this glass vessel on a wooden support b, and is 

 without conical reflector or any similar arrangement for col- 

 lecting the rays. Its upper face, carefully covered with lamp- 

 black, is directed upwards towards the source of heat. The 

 lower face is protected by a packing of cotton-wool against 

 any possible sudden change of temperature. The aperture of 

 the vessel A is closed air-tight by a special vessel B, which 

 carries an arrangement for radiating heat to the thermopile. 

 This consists of a bottom plate of metal c, in " the centre of 

 which there is a round hole in wdiich a metallic tube /, 3*5 

 centimetres in diameter, is soldered. In this tube, at about one 

 fifth of its height, there is a thin metal plate a, which, as well 

 as /, is carefully covered w r ith lamp-black. The distance from 

 a to S was 310 millimetres. Into the bottom plate c a second 

 tube, i, is soldered, into which the india-rubber stopper (which 

 carries the manometer-tube e, the conducting-wires, and the 

 tube r leading to the air-pump) is cemented air-tight. Besides 

 these, the plate c carries a glass ring h, cemented round its 

 outer edge, which is open at both sides and enables a current 

 of water to circulate constantly about the tube I. The whole 

 vessel A was placed in an ice-calorimeter C, as used by 

 Lavoisier and Laplace. The water from the water-tap q flows 

 into the inner space round the vessel A, then flows over into 

 the outer space and runs off by the tube s. This arrangement 

 ensures that the whole of the vessel A shall be surrounded by 

 water always of the same temperature; so that the thermopile 

 is always exposed to equal radiation from the glass walls 

 of A. The tap // for the upper cooling vessel-terminates in a 

 leaden tube which surrounds the tube /, and is pierced with 



