OF KADIAN'T HEAT THROUGH DIFFERENT BOUIES. O 



surface, or that of the thickness of the layers and their internal structure 

 on the quantities of heat which freely pass through them. I have en- 

 deavoured to supply these different omissions, but the undertaking has 

 proved too vast for me, and several parts of it are therefore incomplete. 

 I hope however that I shall be able hereafter to return to these, and 

 to treat them in a manner more satisfactory. 



In the mean time I present to the Academy the results of my first re- 

 searches disposed in two memoirs. That which I offer at present con- 

 tains an account of the method pursued in the measurement of calorific 

 transmission and the application of the method in the case of an unva- 

 rying source acting on bodies of different kinds. In the second I shall 

 explain the facts connected with the succession of the screens and the 

 variation of the sources. 



General Considerations on the Free Transmission of Caloric through 

 Bodies, and tlie Manner of Measuring it by means of the TJiermo- 

 midtiplier. 



We have already observed that a diaphanous screen placed at a cer- 

 tain distance from a calorific source stops a portion of the rays M-hich 

 strike its first surface, while the rest pass freely through. We have re- 

 marked besides that after a certain time the heat stopped at the anterior 

 surface, and accumulated there by successive radiations, passes on from 

 layer to layer till it reaches the other sui'face, whence it begins to ra- 

 diate anew; and that this radiation mingling with the heat which passes 

 through the screen by immediate transmission, prevents its being mea- 

 sured exactly. 



When the screens are liquid, the influence of the conducting power of 

 the layers may always be destroyed if we incessantly renew the matter 

 of the screen by means analogous to the strip of water employed by 

 M. Prevost. But it Avould be always very difficult, and often impossible, 

 to apply this artifice to solid bodies and even to such liquids as can be 

 obtained only in small quantities. In order therefore to attain the same 

 end in a general manner, and to render the experiments in some degree 

 independent of conduction, other means must be employed. 



If we consider with due attention the manner in which the second 

 surface of the interposed plate is heated, and the radiation which results 

 from it, we shall see that the latter possesses properties very different 

 from those that belong to the caloric which is freely transmitted. In 

 order to be satisfied of this, we have only to observe that its action 

 changes with the change of distance between the screen and the source ; 

 a thing which does not happen, even in the slightest degree, to those 

 rays that aic, transmitted freely. In fact, it is with the caloric trans- 

 mitted immediately, as it is with light. 



