140 COLLOIDS IN BIOLOGY AND MEDICINE 



processes of the cells and in the adjustment of the metabolism be- 

 tween cells and their surrounding media. The same is true of 

 cholesterin which is frequently associated with them. 



Fats and oils are not soluble in Water and aqueous solutions; but 

 instead they are easily emulsified by a great variety of substances. 

 A few drops of lye suffice to make the finest sort of subdivision of 

 oil in water. It is still an open question, whether this is accomplished 

 by the lye itself, or whether it is due primarily to soaps, which are 

 formed from the free fatty acids always present in fats and oils, and 

 which themselves act as emulsifiers. Soluble soaps, i.e., the fatty 

 acid salts of the alkalis, possess remarkable fat-emulsifying proper- 

 ties; this property is also shared by the intestinal juice, the pan- 

 creatic juice and the bile. Emulsions of fat and oil usually occur 

 in alkaline solution, while on the other hand acids produce floccula- 

 tion. There are exceptions to this, e.g., the lipase of the castor 

 bean emulsifies fat in acid solution , and milk curded by rennet yields 

 a stable acid emulsion on digestion in pepsin-hydrochloric acid. In 

 general, fat emulsions behave like hydrophile colloids; they are not as 

 easily coagulated by neutral salts as are hydrophobe colloids or other 

 suspensions. 



Milk is a natural emulsion of fat (see p. 345 et seq.). 



Though in the examples given so far, fat has been the dispersed 

 phase and water or the aqueous solution the dispersing medium, 

 conversely, water and aqueous solutions may be incorporated in fats. 

 In this case fat is the dispersing medium and the aqueous solution 

 the dispersed phase. Instances of this condition are butter, cold 

 cream, which is cooling because of the water it contains, lanolin, as 

 well as many salves and liniments. Structures like cream and whipped 

 cream occupy a characteristic intermediate position. 



Lecithin behaves in a very peculiar way. It forms an emulsion 

 with water of its own accord; indeed like a protein it swells up in 

 water into a turbid colloidal solution, without dissolving. It may 

 be said that it occupies a place, in respect to its colloidal properties, 

 between the emulsifiable fats and the hydrophile colloids, closely ap- 

 proaching the latter. 



0. FORGES and E. NEUBATJER* studied its properties by experi- 

 menting upon the coagulation of lecithin emulsions. 



The precipitating action of neutral salts is in a lyotropic series 

 similar to that for acid albumin, in which the greatest effect is pro- 

 duced by the anions. Salts of the alkaline earths and the heavy 

 metals frequently yield "zones of inhibition" as described on page 



77 



84. It is remarkable that neither HgCU nor Hg(CN) 2 even in = 







