Prof. Forbes on the Polarization of Heat. 349 



diately] ; that in the change of form of a molecule, whatever 

 may remain, at the distance at which it is situate from the neigh- 

 bouring molecule, of the attractive and repulsive forces of the 

 atoms of which the two molecules are composed, is susceptible 

 of experiencing any changes which tend to make the atoms of 

 the second molecule vibrate. But this manner of considering 

 the subject requiring calculations which I have not made, I 

 have not thought proper to insist on the development of the 

 consequences of this idea. My object in these considerations 

 is only to demonstrate how the vibrations by which heat is 

 propagated in bodies may follow a law entirely different from 

 that of the vibrations of sound, of light and of radiant heat, 

 these latter vibrations being propagated by undulations which 

 leave at rest the part of the vibrating body where they have 

 passed, without a trace remaining of the passage, while the 

 former are made gradually, and from one object to the nearest, 

 and in such a manner that the vibrations of those parts which 

 are nearer to the source of heat remain always superior in 

 intensity to the vibrations more distant, by a quantity which 

 truly diminishes continually and by degrees, but that, mathe- 

 matically speaking, would only become nothing after an infi- 

 nite lapse of time. 



XXXIX. Note relative to the Polarization of Heat. By 

 J. D. Forbes, Esq., F.B.SS. L. 8? E., Professor of Natural 

 Philosophy in the University of Edinburgh. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 



T> Y the Report of Professor Powell's communication to the 

 ■— British Association at Dublin on the subject of radiant 

 heat, which appears in your Journal for last month (p. 296), 

 it seems to me that a doubt is thrown upon one case of the po- 

 larization of heat which I have described, — a doubt which I 

 will endeavour to remove with as much brevity as possible. 



The experiment is this. Two bundles of mica plates are 

 prepared as I have described in my paper*, and each is attached 

 by wax to a sole of wood, so as to sustain it at the polarizing 

 angle when the sole lies flat. When two such bundles are 

 interposed as shown on page 3.51 (fig. 1.), between the source 

 of heat S, and the thermo-electric pile P, more heat reaches 

 the pile than when one of the bundles is raised on edo-e so 

 as to be crossed relatively to the other, as in fig. 2. 



It may be argued, that in these two positions the plates 



[* See Lond. and Edinb. Phil. Mag., vol. vi. p. 210.— Edit.] 



