OF RADIANT HEAT THROUGH DIFFERENT EDDIES. 71 



CoiiclusioH. 



I had intended to introduce liere some general reflections on the dif- 

 ferent hypotheses which have been proposed to explain the phaenoniena 

 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. 



Ratliant 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 thatwhich extingxiishes 

 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 info several equal layers, and determine by expe- 

 riment what ratio the quantity lost bears to the quantity incident upon 

 ^■arh of the layers, we find that the loss thus calculated decreases rapidly 



