4 POROUS TEXTURES ABSORB WATER 



a porous body. It is obvious, therefore, that in the 

 interior of a capillary tube or pore, filled with a 

 fluid, only a certain number of the fluid molecules 

 are in contact with the walls of the tube, and at- 

 tracted by them ; while in the middle of the tube, 

 and from thence towards its parietes, fluid molecules 

 must exist which only retain their place in virtue of 

 the attraction which the molecules attracted by the 

 parietes exert on those not so attracted ; that is, 

 by the cohesive attraction of the fluid. 



Liquids flow out of capillary tubes, which are 

 filled with them, only when some other force or 

 cause acts, because capillary attraction cannot pro- 

 duce motion beyond the limits of the solid body 

 which determines the capillary action. 



depends on The penetration of a fluid into the pores of a 

 attraction, porous body, is the result of capillary attraction ; 

 its expulsion can be effected by a mechanical pres- 

 sure; and may be accelerated by increasing this 

 pressure, and by all such causes as diminish the 

 mutual attraction of the fluid molecules, or the 

 attraction of the walls of the pores for those mole- 

 cules. The condition most favourable to the passage 

 of a fluid through the pores of a porous substance 

 under pressure, is when one fluid molecule can be 

 displaced so as to glide away over another. 



The slightest pressure suffices to expel the dis- 

 placeable particles of water from a sponge ; a higher 

 pressure is required to express the same fluid from 

 bibulous paper ; and a pressure much higher still is 



