and Mechanical Condition on Radiant Heat. 303 



sends a greater proportion of heat through rock-salt than any- 

 other known substance. 



In his latest investigation, Knoblauch examined at great length 

 the diathermancy of rock-salt. With his usual acuteness, he 

 points out several possible sources of error, and with his custo- 

 mary skill he neutralizes these sources. His conclusion is the 

 same as that of Melloni, namely, that rock-salt transmits in the 

 same proportion all sorts of rays. On the opposite side we find 

 the experiments of MM. De la Provostaye and Desains, and 

 those of Mr. Balfour Stewart*, both of which are discussed by 

 Knoblauch. He differs from those experimenters, while my 

 results bear them out. Considering the slow augmentation of 

 transmission which the foregoing Tables reveal, and the consi- 

 derable number of bodies whose heat is transmitted in almost 

 the same proportion by rock-salt, it is easy to see that, where the 

 number of radiants is restricted, such a uniformity of transmis- 

 sion might manifest itself as would lead to the conclusion of 

 Melloni and Knoblauch. It was only by the selection and ex- 

 tension of the substances chosen as radiators that the differences 

 were brought out with the distinctness recorded in the foregoing 

 Tables. 



The differences in point of quality and the absence of perfect 

 diathermancy in rock-salt appear more striking when instead of 

 the transmissions we take the absorptions. In the case of the 

 radiation from powdered rock-salt, for example, 37*2 per cent, of 

 the whole radiation is intercepted by the rock-salt plate. Ac- 

 cording to Melloni, between 7 and 8 per cent, of this is lost by- 

 reflexion at the two surfaces of the salt. This would leave in 

 round numbers a true absorption of 30 per cent, by the plate of 

 rock-salt. In the case of black platinum, the absorption simi- 

 larly deduced amounts to only 4 per cent, of the total radiation. 

 Instead, therefore, of the radiation from those two sources being 

 absorbed in the same proportion, the ratio in the one case is more 

 than seven times that in the other. For the sake of illustration 

 here follow a few of the absorptions determined in this way : — 



Table V. — Radiation through Rock-salt 



Source. 



Absorption. 



Black platinum . . 



. 3-7 



Black oxide of iron . 



. 130 



Red oxide of iron . 



. 15-9 



Sugar 



. 173 



Chloride of silver 



. 22-6 



Rock-salt .... 



. 29-9 



* I think the important experiment first executed by Mr. Balfour 



