COLLOIDS 35 



these effocts are due to the high surfiice tension and cohesion afTnity of the rolloids. 

 In all cellular processes accompanied by manifestations of osmotic pressure or 

 difTtision, liowcner, the crystalloids may be considered as almost entirely responsible. 

 Electrical Phenomena. — As colloids do not separate freely into ions when di.s- 

 solved, they do not con(hict electricity apprecial)ly. However, when an electric 

 current is passed throunh water containing colloids in sf)lution, the colloidal par- 

 ticles tend to pass to one ])ole or the other. Most colloids move toward the anode. 

 This phenomenon, cdln phoresis^, is also generally exhibited by suspensions, and 

 hence in this particiUar the colloids resemble suspensions rather than solutions. 

 Ilelmholtz has explained the movement of the suspended particles as due to the 

 accumulation of electrical charges upon the surfaces of two heterogeneous media 

 when in contact. The nature of the charge depends upon both the suspended 

 substance and the fluid; e. g., sulphur or graphite particles siispended in water 

 assume a negative charge and move toward the anode, but when suspended in oil 

 of t\n-pentinc they become positively charged and move toward the cathode. 

 \N'ater has such a high dielectric co7istant that most substances suspended in water 

 become negatively charged as compared 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 solution, and toward the cathode when in acid solu- 

 tion.-'-' This peculiar i)roperty of proteins suggests that perhaps simple surface 

 phenomena do not suffice to account for the electrification of all colloid particles. 

 Knowing the peculiar amphoteric character of proteins, which is probably due to 

 the presence of both NHo and COOH groups in the molecule, we can readily under- 

 stand that in an acid solution the NH2 radicles are combined with the acid,leaving 

 the COOH radicles free. The molecule would then have acid properties, and could 

 dissociate into an acid H ion and a basic or electrically positive colloid ion. The 

 colloid ion would then go toward the negative pole slowly, because of its great 

 size. When a suitable concentration of both ions is produced the proteins will 

 move towards both poles, this concentration being, in the case of serum albumin, 

 H = 10~^ (Michaelis). Living protoplasm behaves in most instances, as if the 

 proteins were acids bound to inorganic cations (Robertson), and is usually stimu- 

 lated at the cathode on the "make" of the current. It is permeable to ions, and 

 the vitality of a tissue is so dependent on the maintenance of normal permeability 

 that the permeability may be employed as a sensitive and reliable indicator of 

 its vitality (Osterhout-^). This maj^ be done by determining the electrical resis- 

 tance of the cells, which is lowered by anything that lowers their vitality. 



Surface tension,-^ which may be described as the force ivilh which a fluid is 

 striving to reduce its free surface to a minimum, is highly exhibited by colloids as 

 compared with crystalloids. The formation of emulsions and the spreading out 

 of oil upon the surface of water depend upon surface tension. Ameboid movement 

 may be attributed to changes in surface tension, as also may phagocytosis. (The 

 relation of surface tension to these processes will be considered under the subject 

 of Inflammation.) 



The effect of colloids upon chemical processes going on within 

 their solutions or gels is surprisingly small. Salts in solution in a 

 thick gel of agar or gelatin will diffuse almost as rapidly as in water; 

 they will also ionize as rapidly as in watery solutions, and chemical 

 reactions occur with nearly the same speed and completeness as if the 

 colloids were absent. Furthermore it makes little difference whether 

 these processes are measured in a colloidal solution that is liquid, or 

 after it has set in the gel form. These facts merely indicate that the 

 colloids do not greatly impede the movements of molecules or ions in 



"According to Field and Teague (Jom-. Exper. Med., 1907 (9), 222), native 

 proteins in serum move towards the cathode, no matter what the reaction. 



-' Science, 1914 (40), 488. 



^^ See article on "Sm-face Tension and Vital Phenomena," by Macallum, Ergeb- 

 nissed. Physiol., 1911 (11), G02. 



