54: M. MELLONI ON THE IMMEDIATE TRANSMISSION 
We shall presently see yet more striking analogies between the two 
classes of phenomena 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 which 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 who 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 possessing 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+. 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 thermometer 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 different parts. It has been said that the 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 experiment is no 
20 as the quantity transmitted by each of the screens. If the source gave 10a, 
10 6, and 80 c, the transmission would be 10 and the interception 90. Thus two- 
-substances exposed to different radiations may furnish calorific transmissions not 
only varying according to the same order of decrement, but equal inall their periods 
of variation, although the rays emerging from each may be of a different kind. 
* Scheele, Zraité del’ Air et du Feu, Paris, 1778, § 56. 
+ W. Herschel and Brande, Philosophical Transactions for 1800 and 1820. 
{ Philosophical Transaetions, vol. evi. 
