280 PRINCIPLES OF THE MECHANICAL THEORY OF HEAT. 



THE THERMIC MOLECULAR MOTION. 



From the equivalency of heat and labor it undoubtedly results that every 

 development of lM?at by mechanical means must be regarded as the trans- 

 formation of a hodily tuotion into a molecular tnofion; and, conversely, that 

 every performance of work must be considered a transformation of the molecular 

 motion into a hodily motion. This view constitutes the starting point of. the 

 mechanical theory of heat, of which the most essential principles, together with 

 some of the most important consequences springing from them, have been dis- 

 cussed in the preceding paragraphs, without reference, however, to the concep- 

 tion which we must form of this molecular movement, whose results are the 

 different phenomena of heat. 



For the completion of the mechanical theory there is certainl}' needed an 

 hypothesis respecting the nature of this molecular action, although many import- 

 ant questions may be and have been solved without one. In the mean time the 

 construction of such an hypothesis' has exercised the ingenuity of different physi- 

 cists, especially of Clausius, Kronig, and lledtenbacher. 



Kronig and Clausius (Fogg. Annal., xcix and c) suppose that the minute 

 molecules of gases and vapors, mere points in proportion to tlie intervals which 

 separate them, move on with a constant velocity, in right lines, until they 

 impinge against another molecule of the same nature, or against some object to 

 them impenetrable. The pressure of gases against a solid surface is supposed 

 to result from the fact that the molecules in great number continually impinge 

 against the resisting surface and reboimd from it. By an increase of tempera- 

 ture the velocity with which the molecules move is augmented ; and, in line, 

 the temperature is assumed to be proportional to the square of that velocity. 



In the case of solid bodies, the molecules oscillate about a permanent point of 

 equilibrium ; while iu the case of fluids this point of equilibrium does not exist ; 

 but the molecules, notwithstanding their constant and manifold movements, are 

 restrained to determined distances, and cannot, like the gases, move freely apart 

 from one another. 



While the savants just named seek the causes of the phenomena of heat in a 

 movement of the atoms themselves of bodies, lledtenbacher considers the oscilla- 

 tions of the atoms of the ether enveloping the atoms of a body to be the source 

 of those phenomena, as he has explained in his "Dynamiden system," (Maii- 

 heim, 1857.) 



A circumstance which speaks with luuch force in favor of the views of Kronig 

 aild Clausius is that through these views the difference between radiant and 

 sensible heat admits of easy explanation. The former would thus appear to be 

 propagated in a precisely identical manner with the rays of light, by a undulatory 

 movement of the ether, while a vibratory movement of the ponderable atoms of 

 the body would be the source of sensible heat. 



The development of the mechanical theory of heat has, within a fejv 3'^ears, 

 made such progress that it must soon'stand iu the same grade with the undulatory 

 theory of light. In proportion, at the same time, as the mathematical theory is 

 advanced towards completion, will it become more and more practicable to reduce 

 the explanation of the particular phenomena of heat, in a generally intelligible 

 form, to the principles of the mechanical theory. 



