470 



SCIENCE 



[N. S. Vol. XLIII. No. 1109 



bind all the water. The concentrated soaps 

 show a high degree of water-absorbing power 

 and so are among the best emulsifying agents. 

 Very good, too, are blood-albumin, casein, egg- 

 white and egg-yolk, this last already repre- 

 senting an emulsion of oil in a hydrated pro- 

 tein. Good emulsions may also be prepared 

 with aleuronat, and, when the temperature is 

 properly controlled, with gelatin. Not only 

 may proteins be thus used, but various hy- 

 dratable carbohydrates do well. Acacia has 

 long been so used. Starch, dextrin (or the 

 dextrinized starches used in baby foods), and, 

 when the temperature is properly regulated, 

 agar, also serve well. Oil can also be main- 

 tained in finely subdivided form in cane sugar 

 solutions or glycerin, but these emulsions 

 slowly separate. 



The enumerated substances do not all act 

 equally well. This is because, in the produc- 

 tion of a hydrated colloid, they behave differ- 

 ently from both a qualitative and quantitative 

 viewpoint. Best results are obtained with 

 those substances which not only have the 

 power of taking up much water, but which 

 yield ' liquids of good viscosity with all 

 amounts of water that may be added to them. 

 What is wanted is a relatively homogeneous 

 liquid of good tenacity, by which is meant 

 one that possesses good covering power to- 

 gether with great cohesiveuess. 



The action of casein as a stabilizing agent 

 is particularly instructive. Neutral casein 

 does not absorb much water and it does not in 

 this form serve for the preparation of an 

 emulsion. But when alkali is added, it de- 

 velops marked hydrophilic properties, on the 

 appearance of which it becomes one of the 

 best stabilizing agents for emulsions known. 

 It might be thought that the alkali element is 

 so important because it forms a soap in con- 

 tact with oil, and soap has long been known 

 as an effective emulsifier. While some such 

 action no doubt occurs, it is easily proved that 

 the development of hydrophilic properties by 

 the casein is of first importance because acid 

 (which when added to neutral casein converts 

 it into a hydrophilic colloid) works quite as 

 effectively as does alkali. 



An emulsion hreaks whenever the hydro- 

 philic (lyophilic) colloid which holds the 

 aqueous dispersion means is either diluted be- 

 yond the point at which it can taJce up all the 

 offered water, or is so influenced hy external 

 conditions that its original capacity for hold- 

 ing water is sufficiently reduced. 



Certain emulsions, as those of oil in soap, 

 therefore, tend to break on simple dilution. 

 But agents which dehydrate the hydrophilic 

 colloid act even more rapidly and effectively. 

 What will prove to be effective agents in this 

 regard depends, of course, upon the character 

 of the hydrophilic colloid stabilizing the emul- 

 sion. Wlien alkali-casein is used, the addi- 

 tion of acid breaks the emulsion, while alkali 

 will break an emulsion stabilized by acid- 

 casein. The same concentration of acid or 

 alkali is without effect upon an emulsion sta- 

 bilized by a carbohydrate like acacia, or dex- 

 trin. Since even neutral salts will dehydrate 

 an acid- or alkali-protein, they readily serve 

 to break emulsions stabilized by these sub- 

 stances. An emulsion of oil stabilized in soap 

 is readily broken not only by acids and various 

 salts, but also by alcohol. Ether, on the other 

 hand, is relatively ineffective. Practically all 

 these substances in low concentrations are 

 without effect upon emulsions stabilized in 

 hydrated carbohydrates. 



The fact that alcohol and ether are by them- 

 selves thus relatively ineffective in breaking 

 emulsions explains why the ordinary fat ex- 

 traction methods are so often only partially 

 effective in getting the fat out of biological 

 materials, and why previous treatment of the 

 material, as by digestion with strong acids or 

 alkalies and by similar methods, yields higher 

 fat figures than extraction with ether or allied 

 materials alone. 



IV 



The problem of the distribution of fat in 

 living cells or in various secretions from the 

 living tissues may be separated into two divi- 

 sions; first, a chemical one dealing with such 

 questions as that of the origin and transport 

 of fat, and second, a physical one asking, for 



