46 INTRODUCTION 



cannot permeate diffusion membranes. For example, if a stick 

 of agar jelly be placed in a solution of ammoniated copper sul- 

 phate (a crystalloid), and another be placed in *a solution of 

 Prussian blue (a colloid), it will be found that the copper solu- 

 tion penetrates the agar rapidly, while the colloidal solution of 

 Prussian blue does not penetrate it at all. This property is of 

 great importance, undoubtedly, in keeping different colloidal 

 constituents of the cell in given localities within its protoplasm, 

 e. g. y the oxidizing ferments seem to be chiefly localized within 

 the nucleus ; the colloidal glycogen remains where it is formed 

 in the cytoplasm, unable to escape from the cell, whereas the 

 crystalloidal sugar from which it is formed and into which it is 

 converted, diffuses rapidly into or out of the cell. 



The osmotic pressure of the colloids is so small that 

 some investigators doubt that colloids really do exert any 

 osmotic pressure by themselves. They would explain such 

 small positive results as have been obtained by assuming the 

 presence of contaminating substances, a criticism that is well 

 grounded on the fact of the extreme difficulty of obtaining col- 

 loids in a pure state. The closely related phenomena of diffu- 

 sion, depression of freezing-point, and elevation of boiling-pointy 

 are also exhibited by colloids to but an extremely slight degree. 

 For example, in one experiment, the dissolving of from 14 per 

 cent, to 44 per cent, of egg-albumin in water lowered the freez- 

 ing-point but 0.02 to 0.06 ; and some other colloids have 

 even less effect. But the results of the latest and best experi- 

 ments seem to indicate that the trifling effects of colloids upon 

 osmotic pressure and upon freezing- and boiling-points observed 

 in colloidal solutions are due to the colloids themselves, although 

 it may possibly be that some of these effects are due to the 

 high surface tension and cohesion affinity of the colloids. In 

 all cellular processes accompanied by manifestations of osmotic 

 pressure or diffusion, however, the crystalloids may be consid- 

 ered as almost entirely responsible. 



Electrical Phenomena. As colloids do not separate freely 

 into ions when dissolved, they do not conduct electricity appre- 

 ciably. However, when an electric current is passed through 

 water containing colloids in solution, the colloidal particles tend 

 to pass to one pole or the other. Most colloids move toward 

 the anode. This phenomenon, cataphoresis, is also generally 

 exhibited by suspensions, and hence in this particular the colloids 

 resemble suspensions rather than solutions. Helmholtz has 

 explained the movement of the suspended particles as due to 

 the accumulation of electrical charges upon the surfaces of two 



