148 BULLETIN OF THE 



ous molecule and the internal revolutions about its cTenter of inertia 

 present a new difficulty of conception as to the constitution and ac- 

 tion of the setherial medium. For while the molecule (a mere 

 cluster of atoms) is supposed to be flying freely about without ob- 

 struction or retardation, (in order to fulfil the laws of Charles, and 

 of Boyle and Mariotte, ) the individual atoms themselves experience 

 a very considerable resistance to their revolutions ; — the precise 

 measure of which resistance is the kinetic energy absorbed and ex- 

 pended by setherial undulations. And so it results conversely, that 

 if the motion of the sether-waves exceeds that of the molecular 

 atoms exposed to their action, the difference of momentum is taken 

 up by the latter, and through exchanges at molecular encounters is 

 equalized by corresponding increments of velocity in the molecules 

 themselves. Such is the process in all terrestrial heating by solar 

 radiation. And this brings directly to view one important distinc- 

 tion between heat and light, — to wit, that while both are radiated 

 in precisely the same manner, " conduction " has no existence in 

 optical action. The only approach to any such effect in light, is 

 found in the obscure and puzzling phenomena of fluorescence and 

 phosphorescence, and of animal luminosity. In the case of heat we 

 may have a transfer by radiation — always the result of atomic 

 motion, by conduction — always the result of molecular motion, or 

 by convection — always the result of mass motion. 



During the time of a mean free excursion of gaseous molecules 

 at the temperature of incandescence, the atomic periods would per- 

 mit from ten to twenty thousand revolutions. But from the great 

 amount of energy absorbed by the aether it does not appear probable 

 that any considerable portion of such orbital movement can con- 

 tinue throughout the interval of a mean free path. If then it be 

 true that in a majority of the molecular excursions the whole inter- 

 nal atomic motion is absorbed and destroyed, to be renewed again 

 only by the succeeding collisions, there is a constant drain upon the 

 molecular momentum ; a condition which must alike prevail, how- 

 ever low may be the temperature of the gas. While there is thus 

 a constant tendency to equalization of the orbital atomic momen- 

 tum and the rectilinear molecular momentum, the total kinetic 

 energy of the former has been estimated at not more than from two- 

 thirds to three-fourths of the kinetic energy of the latter. 



It is in the gaseous spectrum alone — that is, in the atomic motions 

 of discrete molecules, that perfect uniformity of period, or as we 



