38 THE CHEMISTRY AND PHYSICS OF THE CELL 



a stick of agar jelly be placed in a solution of aminoniated cop- 

 per sulphate (a crystalloid), and another be placed in a solution of 

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

 penetrates the agar completely before the colloidal solution of Prus- 

 sian blue has penetrated it at all. This property is of great im- 

 portance, undoubtedly, in keeping different colloidal constituents of 

 the cell in given localities within its protoplasm, e. g., the oxidizing 

 ferments seem to be chiefly localized within the nucleus; the colloidal 

 glj'cogen 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, dififuses rapidlj' into or out of 

 the cell. 



The osmotic pressure of the colloids is extremely small. The closely 

 related phenomena of diffusion, depression of freezing -point, and ele- 

 vation of hoiUng-point, are also exhibited by colloids to but an ex- 

 tremely slight degree. For example, in one experiment, the dissolv- 

 ing of from 14 per cent, to 44 per cent, of egg-albumin in water low- 

 ered the freezing-point but 0.02° to 0.06° ; and some other colloids 

 have even less effect. The results of the latest and best experiments 

 seem to indicate that the trifling effects of colloids upon osmotic pres- 

 sure and upon freezing- and boiling-points observed in colloidal solu- 

 tions are due to the colloids themselves, rather than to included im- 

 purities, 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 dififusion, however, the crystalloids may be considered 

 as almost entirely responsible. 



Electrical Phenomena. — As colloids do not separate freely into ions 

 when dissolved, they do not conduct electricity appreciably. How- 

 ever, 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. 

 eataphoresis, 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 accunmlation of electrical charges u])on the surfaces of two 

 heterogeneous media when in contact. The nature of the charge de- 

 pends upon both the suspended substance and the fluid ; e. g., sulphur 

 or graphite particles suspended in water assume a negative charge 

 and move toward the anode, but when suspended in oil of turpentine 

 they become positively charged and move toward the cathode. AYater 

 has such a high dielectric constant that most substances suspended in 

 water become negatively charged as eoni])ared with the water, and 

 move toward the positive pole or anode. 



Hardy has observed that colloidal solutions of coagulated proteins 

 move toward the anode when in alkaline solnlion, and toward the 



