110 M. F. Mohr on the Nature of Heat. 



reaching its extreme position, the latter communicates its 

 charge to the insulated arm m by the brass ball d, and its ne- 

 gative electricity is thus distributed over the surface of A next 

 to B and thus augments its original charge. At the end of its 

 path B / is momentarily connected to earth. It will be evident 

 that on again bringing the plates into contact the charge in B 

 is augmented ; also that if a supply of negative electricity is 

 required, the only modification of the above is to give to in a 

 charge of positive instead of negative electricity. 



The above instrument forms an easily constructed and worked 

 electrical machine, and is beside interesting as rendering auto- 

 matic the classical electrophorus of Volta. 



XIV. Views of the Nature of Heat. By F. Mohr.*. 



T'-HE phenomena of heat have been till now almost exclu- 

 sively explained in text-books by the assumption of a 

 Heat-substance. The discoveries of Melloni have made this 

 view inapplicable to the phenomena of Radiant Heat ; they 

 require the assumption of vibrations similar to those of the 

 Undulatory Theory of Light. The Propagation, Transmission, 

 and Polarization of Radiant Heat have been completely ex- 

 plained by these assumptions ; and, with such facts to guide us, 

 it is certainly no mere idle speculation to attempt to extend 

 this view to the phenomena of common or stationary heat ; 

 rather it is in the highest degree seasonable to point out how 

 this view, which depends upon the well-settled facts of Melloni, 

 explains with overwhelming clearness most of the phenomena 

 of stationary or conducted heat ; and it is to be expected that, 

 with this reform in our ideas, there will be a corresponding 

 reform in our terminology. 



Heat is thus no longer a particular kind of matter, but an 

 oscillatory motion of the smallest parts of bodies. Radiant 

 Heat is propagated in straight lines ; and the molecules vibrate 

 in all directions in a plane perpendicular to the direction of 

 the ray. A polarized ray vibrates in one direction only in this 

 plane ; on the contrary [the particles of] an ordinary hot 

 body vibrate in all possible directions of space, and therefore 

 propagate their heat uniformly in all directions. The propa- 

 gation of heat by contact is thus a communication of motion 

 by impact ; and cooling is a relative coming to rest. The num- 

 ber of heat-vibrations per hour must, as in the case of light, 

 be very large, since all bodies become luminous at a certain 

 temperature ; but even at this temperature waves of light and 



* Translated by Professor Tait from Liebig'e Annalen, vol. xxiv. 1837. 



