21 M. MELLONI ON THE FREE TRANSMISSION 



Wlience it is concluded that the losses still decrease at a distance of 

 about 100 millimetres. 



To comprehend at a single glance the law of the propagation of ca- 

 loric radiating through diaphanous media we have only to reduce the 

 results contained in the first two columns of the Tables A. and B. to a 

 linear construction. 



The mere inspection of the curves thus constructed shows that the raj's 

 lose verj' considerably when they are entering the first layers of the me- 

 dium. But in proportion to their distance from the surface we see that 

 the loss decreases and that at a certain distance it is almost imperceptible, 

 and the rays seem to continue their progress, retaining all their inten- 

 sity ; so that in glass and in oil of colza, and probably in all other dia- 

 phanous media, the portion of heat which has forced its passage through 

 the first layers must penetrate to very great depths. 



Delaroche had found that the heat which has passed through one 

 plate of glass becomes less subject to absorption when it is passing 

 through a second. The identity of this fact with the law of resistance 

 in continuous media shows that the solution of the continuity and the 

 interposition of the atmosphere between the two screens do not alter 

 the nature of the modifications which the rays undergo in the first plate 

 of glass. It is tlierefore exceedingly probable that the proposition of 

 Delaroche is true with respect to a very numerous series of thin screens ; 

 for we have just seen that in the same medium the losses still diminish to 

 the depth of 80 or 100 millimetres. In reference to this point, the follow- 

 ing is the result of the experiments I have made with four plates of the 

 same glass that had been employed in the first attempts to investigate tlie 

 law of propagation tlnough continuous media The common thickness 

 of these plates was 2°""-068. 



Numbers of Deviations of the galva- 



thc screens. nometer. 



1. 21-62 



2. 18-75 



3. 17-10 

 4>. 15-90 



It is scarcely necessary to observe that the common radiation to which 

 the screens had been exposed was always 30 degrees, answering to a 

 force or temperature of 35-3. If we represent this radiation by 1000, as 

 we have done in all the foregoing cases, we have: 



Numbers of the screens. Rays transmitted. Kays stopped. 



1. 619 381 



2. 531 469 



3. 484 515 



4. 450 540 

 Whence we have 



0-381, 0134. 0'087, 0-058, 



