THE FUNCTIONS OF PROTOPLASM AND CELLS 45 



surface increases the rate of chemical and physical activity at every face 

 of contact between the two phases of the system. These activities have 

 been called surface 'phenomena. Some surface phenomena are surface 

 tension, adsorption, and various electrical phenomena. 



Surface tension is exemplified by the film at the surface of water, the 

 external membranes of cells, the membrane of the nucleus, and the films 

 that surround vacuoles. A considerable pull is exerted by these films. 



Extremely finely divided solids or those with extremely fine pores tend 

 to condense on their surfaces anj^ gases or vapors or other substances 

 with which they are in contact. Such substances are said to be adsorbed. 

 The thin films of these adsorbed substances are held so tenaciously that 

 great pressures are required for their removal. A gas mask removes 

 gases from the air because of the great adsorptive power of charcoal, and 

 the clarification of sirups and sugars is accomplished by making use of 

 the adsorption of coloring matter by bone black. Certain properties of 

 living matter are best explained on the basis of adsorption. 



Electrical properties are conferred on protoplasm by its ionized salts. 

 Ions are capable, as explained in Chap. 3, of conducting electricity but 

 in protoplasm are more important because they are probably adsorbed 

 upon the surfaces of the colloidal particles. These particles thereb}^ 

 acquire an electric charge. Through the interior of the cytosome the 

 particles appear to carry positive charges, but in the nuclear sap they are 

 negative. The surface of a cell as a whole seems, as stated before, to be 

 negatively charged. The occurrence of like charges on the interior 

 particles causes mutual repulsion and is probably the chief reason why 

 these particles do not adhere to one another. If they did adhere, the 

 protoplasm would coagulate or harden. 



Changes in Viscosity. — Viscosity is the resistance which the particles 

 of a substance otfer to movement upon one another. The viscosity of 

 light liquids like water or gasoline is low, while that of thick sirup — or 

 still more so of solids — is high. When a bit of fresh meat is subjected 

 to pressure while still warm, even if it be from an organ which like the 

 liver has no conspicuous fibers in it, it appears to be highly viscous. The 

 resistance is offered mostly, however, by the cell membranes. These are 

 firm enough, like well-filled bags of wheat, to tend to preserve the shape 

 of the cells. The interior protoplasm of a cell, at least of those which 

 have been studied in this respect, turns out to be quite liquid. In one 

 kind of cell the protoplasm is only about ten times as viscous as water 

 and only about one one-hundredth as viscous as ordinary glj^cerin. 



This fluid state is probably maintained, as indicated in the preceding 

 section, by the like electric charges on the colloidal particles in the proto- 

 plasm, causing these particles to repel one another. The viscosity 

 changes frequently, however, for reasons not yet understood. Such 



