COLLOIDS 39 



cathode when in acid solution.^" Tliis peculiar property of proteins 

 sugorests that perhaps simple surface phenomena do not snfifice to ac- 

 count for the electrification of all colloid particles. Knowing the pe- 

 culiar amphoteric character of ])r()tcins, which is probably due to the 

 presence of both NII2 and COOII groups in the molecule, we can 

 readily understand that in an acid solution the NH2 radicles are com- 

 bined with the acid, leaving the COOH iradicles free. The molecule 

 would then have acid properties, and could dissociate into an acid II 

 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 pro- 

 teins will move towards both poles, this concentration being, in the 

 case of serum albumin. H^IO " (^lichaelis). Living protoplasm be- 

 haves in most instances, as if the proteins were acids bound to inor- 

 ganic cations (Robertson), and is usually stimulated 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 may be 

 done by determining the electrical resistance of the cells, which is 

 lowered by anything that lowers their vitality. 



Surface tension,-" which may be described as the force icith tchich a 

 fluid is strii'i)ig 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 de- 

 pend 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 colloid 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 

 solutions. On the other hand, as before mentioned, colloids diffuse 

 very slowly into each other. Hence, in the cell the colloids are quite 

 fixed in their positions, whereas the crystalloids may wander about 

 freely, and this arrangement is certainly of great importance in bio- 



19 According to Field and Teague (Jour. Exper. Med., 1007 (9), 222). vative 

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



ina Science. 1904 (40), 4SS. 



20 See article on "Surface Tension and Vital Phenomena." hv ^Macallum. Erirel)- 

 nisse d. Phvsiol., 1011 (11). (\()2. 



