54: M. iMEJLLOXI ON THE IMMEDIATE TRANSiMlSSION 



We shall presently see yet more striking analogies between the two 

 classes of phaenomena when we consider the modifications which the ca- 

 lorific rays undergo in their passage from one screen to the other. But 

 before we dismiss the present subject it may be advisable to bestow a 

 few moments' attention on the purposes to wliich the calorific proper- 

 ties of rock salt may be applied. 



Glass is a substance but very slightly diathermanous, especially when 

 the temperature of the source is low. The common prisms or convex 

 lenses could not therefore be employed for the purpose of ascertaining 

 whether radiant heat be subject to changes of direction analogous to 

 those of light in penetrating to the interior of refracting media. It was 

 owing to the use of such instruments that some Avho applied themselves 

 to the investigation of this point attained but very indecisive results, and 

 often drew from them very false conclusions. Scheele asserted that 

 " bright points not j^ossessing the least heat may be formed before the 

 fire with burning-glasses*." Carefully conducted experiments have 

 more recently shown that a thermometer rises some degrees when placed 

 in the focus of a lens exposed to the radiation of flame or of incan- 

 descent bodies -j-. But as the heat is then luminous, and as no very de- 

 cided effect is observed if the operation is performed with nonluminous 

 heat, it was inferred that the elevation of temperature was owing to the 

 light absorbed by the tliermometer and that isolated radiant heat is not 

 susceptible of refraction. . This notion might derive additional support 

 from the fact that lenses of rock crystal, Iceland spar, alum, and other 

 diaphanous substances acted analogously to the glass lens : and yet it 

 would have been wrong to attribute to the agent an effect which was due 

 only to the particular structure of all those substances. To be satisfied 

 of this we need only operate with a lens of rock salt ; for the focal ther- 

 mometer then always exhibits a marked elevation of temperature, even 

 though the radiant heat be totally separated from the light. But it has 

 been attempted to explain the effect of the lenses by an inequality in the 

 heating of their diff"erent parts. It has been said that tlve heat is accu- 

 mulated towards the centre, that the parts towards the margin, because 

 of their thinness, quickly grow cold again, and that it is not surprising 

 therefore to see the thermometer rise more rapidly when placed in the 

 prolongation of the axis of the lens than in any other direction:];. It 

 would however still remain to be explained why the expeiiment is no 



20 as tlie quantity transmitted by each of the screens. If the source gave 10 a, 

 10 b, and 80 c, the transmission would be 10 and the interception 90. Thus two 

 substances exposed to difierent radiations may furnish calorific transmissions not 

 only varying according to the same order of decrement, but equal in all their periods 

 of variation, although the rays emerging from each may be of a different kind. 



* Scheele, Trailc de I'Air ct du Feu, Paris, 1778, § 56. 



t W. Uerschel and Brande, Pliilosoiihical Transactions for 1800 and 1820. 



X Philosophical Transactions, vol. cvi. 



