172 Molecules, Ultimates, A totns, tind Waves. [April, 



intrinsic inertia predominates in gases and vapours, the ad- 

 ventitious in solids and liquids. 



Incandescence begins in all solids and liquids at precisely 

 the same temperature. A red heat or a white heat indicates 

 the same temperatures whatever be the nature of the in- 

 candescent substance, provided it be either in the solid 

 or liquid condition. But then it requires very different 

 amounts of applied motive energy to raise different substances 

 to the temperatures indicated by those phenomena of incan- 

 descence. This amount depends on the specific heat of the 

 substance, or the inertia of its particles. Thus the motive 

 energy required to raise water to the temperature of a red 

 heat is about 33 times the amount requisite to raise an equal 

 weight of mercury to the same temperature; yet both 

 substances would exhibit the same tint of redness. Part of 

 this difference may be due to the greater space over which the 

 motive energy is diffused in the case of the water, which has 

 upwards of 13 times the bulk of an equal weight of mercury; 

 but for equal volumes the water still requires about two and 

 a half times more applied energy than does the mercury. 

 Both substances being in. the liquid condition, this difference 

 can be most readily explained on the supposition that the 

 molecules of water have a greater intrinsic inertia than have 

 the ultimates of mercury. 



It is the adventitious inertia, however, that is most pro- 

 minently displayed in the case of incandescent solids and 

 liquids. The difference in the colour of the light which 

 they exhibit is an exact measure of the degree of rapidity of 

 the vibrations communicated to the luminiferous ether by 

 those of the ponderable particles. The two sets of vibrations 

 must be synchronous ; and as we can determine the rate of 

 the ethereal vibrations appertaining to each tint, so we can 

 ascertain the rate of the corresponding vibration in the 

 particle by which the ethereal vibration is generated. 



In all solids and liquids the force of cohesion by which 

 the particles are held together must act on them very un- 

 equally, according to their position in the mass. Those at 

 and near the surface are almost entirely freed from the opera- 

 tion of that force in one direction, while on those within the 

 mass the cohesive attraction acts in all directions. The 

 latter must, therefore, have by virtue of their position a 

 greater amount of immobility than the former, and they will 

 remain at rest after the former have been set in motion by 

 the applied force, or at least they will more slowly acquire 

 their proper rate of vibration. When incandescence begins, 

 it is only the most movable particles— those at and near 



