Prof. J. Tyndall on 



[Dec. 18, 



16 



chamber P is closed at its end by the rock-salt plate t, through which 

 the heat rays from C can reach the posterior face of the pile. 



Adjusting the screen S so that the rays impinging on both sides of 

 the pile accurately neutralise each other, the rock-salt plates r, r' , being 

 first in contact, by means of a screw d, one of the plates r is withdrawn 

 from the other, dry air being introduced into the space between them. 

 The layer of air could be gradually augmented in thickness from zero 

 to 3 inches, which exceeds by more than 50 per cent, the thickness of 

 the layer to which Professor Buff ascribes an absorption of 50 or 60 

 per cent. 



The apparatus is delicately constructed, and to avoid the strain upon 

 the plates of rock-salt r, r', which would occur if the chambers F and 

 P were exhausted, each of these chambers was filled with dry hydro- 

 gen. This, according to Professor Buff, is tantamount to a vacuum, 

 exercising no action whatever on the rays of heat. 



Repeated experiments with this apparatus proved the absorption of 

 the layer of dry air in the chamber i to be nil. 



But here another question arises. The rays of heat in these experi- 

 ments had passed through plates of rock-salt. Supposing, therefore, 

 that air and rock-salt absorb the same rays, the sifting of the rays by 

 the salt would cause the air within the experimental tube to behave as 

 a vacuum. Professor Buff endeavours to prove that this is the case. 

 He first finds the transmission of radiant heat by a plate of clear rock- 

 salt 3 millims. in thickness to be less than 60 per cent, of the entire 

 radiation; aud then he finds that the thermal colours of rock-salt and 

 air are " sufficiently similar to explain the phenomena observed by 

 Tyndall." 



It is hardly credible that the skilled experimenters who preceded 

 Professor Buff should have failed to detect the high absorption which 

 he ascribes to rock-salt, if it were a fact. Melloni's determination 



