124 Mr. S. T. Preston on the Application of the 



why does not some of the heat pass from the gas experimented 

 on to this gas ? In answer to this, it must be kept in view 

 that the gravific medium, though in principle constituted as 

 an ordinary gas, differs from an ordinary gas profoundly in 

 several respects. First, it is necessary to assume that its 

 particles are (as essential to the long free path) incomparably 

 more minute than those of an ordinary gas, and the number 

 of particles in unit of volume much greater. A molecule of 

 an ordinary gas surrounded by the particles of the gravific 

 medium, might be compared (as regards relative dimensions) 

 to a visible mass surrounded by the molecules of air. Next, 

 it is necessary to assume that the velocity of the minute par- 

 ticles of the gravific medium is incomparably greater than that 

 of the relatively massive molecules of ordinary gases. Now, 

 it is a known fact that the resistance to the passage of bodies 

 through a medium constituted according to the kinetic theory 

 diminishes as the normal velocity of the particles of the medium 

 increases. By making, therefore, the normal velocity of the 

 particles of the medium sufficiently great, all perceptible re- 

 sistance to the passage of bodies through it will disappear. 

 It is as if the medium did not exist; it becomes quite impalpable, 

 or its presence impossible to detect. This is consistent with 

 observation. The amount of energy, or motion, abstracted 

 from a body passing through the medium, and given up to 

 the medium, is exactly measured by the resistance encountered 

 by the body. It is this transference of energy to the medium 

 that constitutes the " resistance." If, therefore, there is no 

 measurable resistance to the passage of the body through the 

 medium, there is no measurable energy abstracted from the 

 body. This gets over our difficulty ; for since the molecules 

 of ordinary gases (at their relatively slow velocity) move 

 through the gravific medium without appreciable resistance, 

 there is no perceptible transference of energy (i. e. " heat ") 

 from them to the gravific medium. In other words, the 

 presence of the gravific medium cannot interfere with the ex- 

 periments on the specific heat of ordinary gases. In short, 

 the high normal velocity of the particles of the medium 

 necessarily renders it in all respects completely impalpable, or 

 its presence impossible to detect by the senses. The high 

 velocity of the particles is only naturally adapted to the 

 minute size of the particles. 



14. It would seem difficult to avoid the application of the 

 above principles to the case of molecules in close proximity — 

 " cohesion " or " chemical union." For, first, it would ap- 

 pear obvious that molecules in contact would be urged together 

 with exceptional force, owing to the parts in contact cutting 



