148 PROTOPLASM 



grams of the lyophilic colloid (gelatin) which just fails to prevent 

 a change in color from red to violet when 1 cc. of a 10 per cent 

 solution of sodium chloride is added to 10 cc. of Zsigmondy's 

 (formaldehyde) red colloidal gold. Some gold numbers are 



Gelatin 0.005 to 0.01 Dextrin 10 to 20 



Egg albumen 0.06 to 0.30 Potato starch 25 



Gelatin is the most efficient among the protective colloids; i.e., 

 less of it is needed, because it is the most resistant to electrolytes. 



Findlay has given some examples of the protective action of 

 substances. He tells us that the waters of the Mississippi and 

 of the Nile are always muddy owing to the presence of large 

 amounts of colloidal organic matter which stabilizes the fine 

 suspension of clay. The water of the Ohio river, on the other 

 hand, is clear owing to the absence of protective colloids and the 

 presence of hme and other salts which act as precipitating agents. 



The protective action of organic matter plays an important 

 part in physiological processes. Findlay cites the case of bile 

 albuminoids which act as protective colloids and keep poorly 

 soluble substances, such as cholesterol and the salt of bilirubin, 

 in the colloidal state and thus prevent their deposition as gall- 

 stones until pathological conditions interfere. 



When milk curdles, it is the casein which does so. The process 

 is made less easy by the presence of lactalbumin in milk, which 

 is more resistant than is casein to the electrolytes of the stomach 

 and therefore partially protects the casein from coagulation. 

 Cow's milk is less easily digested than human milk, which is less 

 digestible than ass's milk. The reason probably lies in the 

 relative proportions of casein and lactalbumin in the three cases, 

 there being least lactalbumin in proportion to casein in cow's 

 milk, and most in ass's milk. Proof of the theory lies in the fact 

 that the digestibility of cow's milk can be increased by adding 

 gelatin, white of egg, or barley. Curdling then occurs less readily, 

 owing to increased protection by these colloids. 



The gold number finds application in medicine in the diagnosis 

 of certain diseases. The gold number of the spinal fluid of a 

 normal person is a definite value; any deviation from it is indic- 

 ative of disease. Certain afflictions are thus characterized. 



Structure.— The structure of the nonelastic, or nonswelling, 

 gels is that of a finely porous "sponge," comparable to pumice 



