THE LYMPHATIC SYSTEM. 311 



According to Liebig,* the requisite pressure for different liquids, passing 

 through the same membrane in a given time, is as follows : 



PRESSURE REQUIRED TO CAUSE TRANSUDATION THROUGH OX-BLADDER. 



Kind of liquid. Height of the mercurial column. 



Water 320 millimetres. 



Solution of salt 530 " 



Oil 906 



Alcohol 1280 



The different diffusibility of different substances has been employed 

 for separating them from each other, when mingled in the same solution. 

 This process is termed Dialysis. If a solution containing both gum 

 and sugar be placed on one side of a membranous diaphragm, with 

 pure water on the other, the sugar will pass through, while the gum 

 will be left behind. If a mixture of albumen and sodium chloride be 

 placed under the same conditions, the salt will transude leaving the 

 albumen by itself; the two substances being thus separated by the 

 action of the membrane. By this means poisonous crystallizable matters 

 may be extricated from organic mixtures in sufficient purity for their 

 detection by chemical tests ; and on the other hand albuminous matters 

 may be purified from the saline ingredients of the animal fluids, and 

 obtained in a condition for examination and analysis. 



Endosmosis and Exosmosis. Beside the elimination of chemical 

 ingredients, as above described, transudation often gives rise to a change 

 in volume of the fluids on either side of the membrane. When a mem- 

 brane is interposed between two liquids which are transmitted with 

 different degrees of facility, that which passes most readily will accu- 

 mulate on the opposite side of the membrane. 



If, for example, a solution of salt and an equal volume of distilled 

 water be placed in contact with opposite sides of the membrane, after 

 a time they will have become mingled, to some extent, with each other. 

 A part of the salt will have passed into the water, 'giving it a saline 

 taste ; and a part of the water will have passed into the saline solution, 

 making it more dilute than before. If the quantities of the two 

 liquids be now measured, it will be found that a comparatively large 

 quantity of water has passed into the saline solution, and a compara- 

 tively small quantity of the saline solution has passed into the water. 

 That is, the water passes inward to the salt more rapidly than the 

 salt passes outward to the water. The consequence is, that the volume 

 of the saline solution is increased, while that of the water is dimin- 

 ished. The more abundant passage of the water, through the membrane 

 to the salt, is called endosmosis; and the more scanty passage of the salt 

 outward to the water is called exosmosis. 



The mode usually adopted for measuring the rapidity of endosmosis 

 is to take a glass vessel, wide at the bottom and narrow at the top, 

 with a membrane stretched over its larger orifice and secured by a 



* Annales de Chimie et de Physique. Paris, 1849, tome xxv., p. 373. 



