392 Prof. De Volson Wood on 



10~ 18 of a pound per cubic foot; but arbitrary values are of 

 small account unless checked by actual results. 



We propose to treat the aether as if it conformed to the 

 Kinetic Theory of Gases, and determine its several properties 

 on the conditions that it shall transmit a wave with the velo- 

 city of 186,300 miles per second, and also transmit 133 foot- 

 pounds of energy per second per square foot. This is equivalent 

 to considering it as gaseous in its nature, and at once compels 

 us to consider it as molecular ; and, indeed, it is difficult to 

 conceive of a medium transmitting light and energy without 

 being molecular. The Electromagnetic Theory of Light sug- 

 gested by Maxwell, as well as the views of Newton, Thomson, 

 Herschel, Preston, and others, are all in keeping with the 

 molecular hypothesis. If the properties which we find by 

 this analysis are not those of the aether, we shall at least 

 have determined the properties of a substance which might 

 be substituted for the aether, and secure the two results 

 already named. It may be asked, Can the Kinetic theory, 

 which is applicable to gases in which waves are propagated 

 by a to-and-fro motion of the particles, be applicable to a 

 medium in which the particles have a transverse move- 

 ment,- whether rectilinear, circular, elliptical, or irregular? 

 In favour of such an application, it may be stated that 

 the general formulae of analysis by which wave-motion in 

 general, and refraction, reflection, and polarization in parti- 

 cular, are discussed, are fundamentally the same : and in the 

 establishment of the equations the only hypothesis in regard 

 to the path of a particle is — It will move along the path of 

 least resistance. The expression V 2 oc e-r-8 is generally true 

 for all elastic media, regardless of the path of the individual 

 molecules. Indeed, granting the molecular constitution of 

 the aether, is it not probable thai the Kinetic theory applies 

 more rigidly to the aether than to the most perfect of the 

 known gases ? * 



The 133 foot-pounds of energy per second is the solar-heat 

 energy in a prism whose base is 1 square foot and altitude 

 186,300 miles, the distance passed over by a ray in one 

 second ; hence the energy in 1 cubic foot will be 



l8p005T6280 = BlTlO' foot-pounds. . . (1) 



Where results are given in tenth-units of high order, as 

 in the last expression, it seems an unnecessary refinement to 



* See also remarks by G. J. Stone y, Phil. Mag. 1868 [4] xxxvi. 

 pp. 132, 1:53. 



