3(54 CAPILLARY ATTRACTION, ETC. [MEMOIR XXVI. 



If, as it thus appears, no pressure we can command be 

 sufficient to restrain one gas from passing into another, 

 we next inquire what obstacle the condensed gas pre- 

 sents. There is abundant evidence to show that this 

 medium bears the same relation to the percolating gas 

 that a vacuum would do, inasmuch as the rate of dis- 

 charge into it is the same as it is into a vacuum. If the 

 particles of different gases possess no repulsive tendency 

 as respects each other, if the presence of one makes no 

 difference in, nor produces any retardation in, the par- 

 ticles of the other, then it is immaterial how many of 

 such particles are condensed together in a given space. 

 The vacuum is not less a vacuum because it is contained 

 under smaller dimensions, any more than a torricellian 

 vacuum is less perfect when the mercury is made to rise 

 nearly to the top of a barometer tube than it was when 

 there was a vacant space several inches in length. This 

 would therefore indicate that these diffusions will take 

 place under all pressures, provided the gaseous condition 

 subsists; and this conclusion is abundantly borne out 

 by the experiments herein detailed. 



The explanation we thus give of the action of con- 

 densing barriers rests upon a fundamental principle of 

 dynamics, that when the moving force and the matter 

 to be moved vary in the same proportion, the resulting 

 velocity will always be the same. Thus, if a cylinder 

 filled with air and fitted with a piston communicate with 

 a vacuum through an aperture, it is immaterial whether 

 the air be allowed to flow into the void without any 

 pressure, or whether it be urged by a direct action on 

 the piston its velocity, as it goes into the void, will be 

 the same in both cases ; for if it be compressed, the ac- 

 tion of the piston is to reduce the air to such a density 

 that its elasticity is equal to the compressing force ; and 

 because the elasticity varies as the density, the density 



