IS EQUIVALENT TO A MECHANICAL PRESSURE. 53 



held in equilibrium by the column of mercury, and 

 the result is that exactly as much brine flows 

 out as water flows in. 



Let us suppose the column of mercury to be re- Additional 



. . experiment. 



moved, and the rise of the brine in the narrow tube 

 is explained at once. If we close a short tube, 

 filled with alcohol or brine, with bladder at both 

 ends (an arrangement which may represent a cell), 

 and suspend it in a vessel of pure water, both sur- 

 faces of the bladder become convex outwards ; they 

 sw r ell, but without bursting. As soon as the pres- 

 sure, gradually increasing by the influx of water 

 into the interior of the tube, is sufficient to keep in 

 equilibrium the affinity of the water for the blad- 

 der, and consequently its further influx, the ex- 

 change goes on, for the future, without change of 

 volume. 



Most porous bodies exhibit the phenomena de- Porous 



t bodies in 



scribed in the preceding pages, if their pores are so general 



exhibit 



minute that a feeble hydrostatic pressure is not pro- similar 



phenomena. 



pagated through them. Ihese phenomena may be 

 produced with clay cells* (such as are used for gal- 



* I consider it of sufficient importance to state here that s n ab f 

 porous clay also takes up unequal volumes of brine and water, sorbed by 

 In special experiments made on this subject, cells of clay (mode- baked 8 clay, 

 rately ignited porcelain biscuit) were laid for 24 hours in pure 

 water, then carefully dried externally with bibulous paper, and 

 the increase in weight, that is, the weight of the absorbed water, 

 carefully determined. The clay was then carefully dried, laid 

 for 24 hours in brine, and the weight of the absorbed brine 

 determined in like manner. In a second series of experiments, 



