INFLUENCE OF MEMBRANES UPON OSMOTIC CURRENTS. 479 



be necessary to pass in review the laws which regulate these 

 phenomena. This is all the more necessary, as in most works 

 upon physics, the physiological application of these laws are 

 not fully appreciated ; while, on the other handj physiol- 

 ogists are too apt to neglect physical explanations of the so- 

 called vital phenomena. 



Capillary attraction is dependent upon an adhesive force 

 or attraction between certain surfaces and certain liquids. A 

 liquid, in order to be subject to this force, must be capable 

 of wetting the surface to which it is exposed. This is illus- 

 trated by the well-known attraction between water and 

 glass. If a drop of water be placed upon a clean glass plate, 

 it becomes flattened and adheres to the surface ; and when 

 the plate is inclined, runs to the edge, leaving a wet line, and 

 does not fall to the ground, but adheres to the plate and is 

 sustained against the force of gravity. If the plate be care- 

 fully covered with grease, the drop of water will not adhere 

 to the surface, and is more nearly spherical ; and when the 

 plate is inclined, it rolls to the edge and falls to the ground. 

 This is explained by the fact that water is not capable of 

 wetting a greased surface. If a globule of mercury be placed 

 upon the glass plate, instead of water, it moves about freely, 

 and falls to the ground, because no attraction exists between 

 it and the surface. 



If the experiment be now modified by partly immersing 

 a clean glass plate in a vessel of water, another set of phenom- 

 ena is presented. At the point where the glass is in contact 

 with the surface of the liquid, a slight elevation of water along 

 the sides of the glass will be observed, which is in opposition 

 to both the force of gravitation and the cohesive molecular 

 attraction between the particles of the liquid. No argument 

 is necessary to show that, within certain limits imposed by 

 the law of gravitation, the ascent of liquids thus attracted by 

 the surfaces of solids will bear a direct ratio to the predomi- 

 nance of the adhesive attraction between the surface and the 

 liquid over the cohesive attraction between the molecules of 



