PHYSICAL PKOCESSES IN CELLS. 43 



walls of the tube will then intersect, and the surface of the fluid within 

 the tube will be a concave meniscus. In other words, every portion of 

 the surface of the liquid within the tube will be under the attractive influ- 

 ence of the walls of the tube. A certain portion of the fluid within the 

 tube will so be held up by adhesion to the tube, and will hence exert 

 no downward pressure. As a consequence the downward pressure within 

 the tube will be less than the upward pressure of a column of fluid of 

 the same height without the tube. Any molecule on any plane below 

 the surface of the fluid in the tube would so be subjected to two 

 unequal pressures, a greater upward pressure and a lesser downward 

 pressure ; the column of liquid will therefore rise within the tube until 

 these two pressures are equal. 



When, however, the force of cohesion of the liquid is greater than 

 that of adhesion to the walls of the tube, as already explained, the sur- 

 face becomes convex and the surface tension is increased. Since the 

 molecular forces are greater than gravity, the downward pressure in the 

 tube is greater than the upward pressure to which any plane is subjected 

 by the weight of the liquid outside of the tube. The fluid then is de- 

 pressed in the tube until these two pressures are equal. 



Capillarity partly explains the ascent of the sap in trees, the ascent 

 of oil in a lamp-wick, to a certain extent the movement of the blood and 

 lymph in the capillaries, but more especially the entrance of fluid into 

 porous bodies, — a fact of the greatest importance as underlying the expla- 

 nation of imbibition, filtration, and osmosis. i 



4. Solution. — That a substance may enter the interior of cells it 

 must, as a rule, be in a state of solution; though we shall find, when we 

 study the process of absorption, that there are several exceptions to this 

 statement. 



The process of solution of solids in fluids is of very general occur- 

 rence in cell life. Almost all food-stuffs are solid, and to be of nutritive 

 value must first be reduced to the form of a solution ; even the con- 

 sumption of organic matter in the vital processes of a cell results in the 

 formation of a watery solution, as in the formation of urine, sweat, and 

 the various secretions. 



When a solid dissolves in a liquid, the cohesion of the molecules 

 of the solid is broken by their adhesion for the molecules of the liquid. 

 When, therefore, the attraction of the liquid for the solid is greater 

 than the cohesion of the solid, the latter is said to be soluble and its 

 molecules separate. The limit of solubility is reached when the attrac- 

 tions of adhesion and cohesion are balanced. 



Anything that reduces the cohesion of a solid favors its solution ; 

 thus, heat accelerates solution by separating solid molecules through 

 the expansion which it produces, and, by increasing the distance between 



