OF RADIANT HEAT THROUGH DIFFERENT EODIES. 71 
Conclusion. 
I had intended to introduce here some general reflections on the dif- 
ferent hypotheses which have been proposed to explain the phenomena 
of heat, and on the question of the identity of radiant heat and light. 
But as these two agents are nowhere more intimately united than in 
the rays of the sun, such considerations should be preceded by a to- 
lerably complete statement of the numerical results obtained by the 
application of our several processes to solar heat. The experiments 
however which I have hitherto been able to make with this view are too 
deficient in number and variety to justify my attempting any statement 
of the kind. I will therefore not enter, for the present, into any dis- 
sertation on the nature of heat, but will conclude with a recapitulation 
of the principal consequences to which I have been led by my inquiries 
into the properties of the radiant heat emitted by terrestrial sources, in 
order that being thus comprehended at a single glance they may be 
more easily compared with the analogous properties of light. 
Radiant heat passes instantaneously, and in greater or less quantities, 
through a certain class of bodies, solid as well as liquid. This class does 
not consist exactly of diaphanous substances, since opake plates or plates 
possessing but a feeble transparency are more diathermanous or perme- 
able to radiant heat than other plates possessing perfect transparency. 
There are different species of calorific rays. They are all emitted 
simultaneously and in different proportions by flame, but in the heat from 
other sources some of them are always wanting. 
Rock salt reduced to a plate and successively exposed to radiations 
of the same force from different sources always transmits immediately 
the same quantity of heat. A plate of any other diathermanous sub- 
stance will, under the same circumstances, transmit quantities less con- 
siderable in proportion as the temperature of the source is less elevated: 
but the differences between one transmission and another decrease as the 
plate on which we operate is more attenuated. Whence it follows that 
the calorific rays from different sources are intercepted in a greater or 
less quantity, not at the surface and in virtue of an absorbent power va- 
rying with the temperature of the source, but in the very interior of the 
plate and in virtue of an absorbent force similar to that which extinguishes 
certain species of light in a coloured medium. 
The same conclusion is attained by considering the losses which the 
calorific radiation from a source at a high temperature undergoes in 
passing through the successive elements which constitute a thick plate 
of any other diathermanous substance than rock salt. For if we ima- 
gine the plate divided into several equal layers, and determine by expe- 
riment what ratio the quantity lost bears to the quantity incident upon 
each of the layers, we find that the loss thus calculated decreases rapidly 
