THE FATS 61 



tion. giving the alkaline salt of a fatty acid and glycerin. The former 

 compound is spoken of as a soap. In water the soaps form a sort of 

 pseudo-solution on heating which sets to a solid jelly on cooling. 

 From a dilute watery solution the soap can be thrown down in the 

 solid form by the addition of neutral salts. Fats are insoluble in 

 and non-miscible with water. If shaken up with water the droplets 

 rapidly run together and rise to the surface, forming a continuous 

 layer of the oil or fat. The same thing happens if an absolutely 

 neutral fat be shaken up with a dilute solution of sodium carbonate. 

 If, however, the fat be slightly rancid, i.e. if fatty acid be present, 

 the latter combines with the alkali with the expulsion of C0 2 to 

 form a soap. The presence of soap in colloidal solution in the water 

 at once diminishes or abolishes the surface tension between the 

 neutral fat and the water. Like many other colloidal solutions, a 

 soap solution presents the phenomenon of surface aggregation, i.e. 

 the concentration of the soap at the surface is increased to such an 

 extent as to form practically a solid pellicle of molecular dimensions 

 on the surface of the fluid. The same pellicle formation occurs at 

 the surface of every oil globule, so that on shaking up rancid oil with 

 dilute sodium carbonate, the whole of the oil is broken up into fine 

 droplets which show no tendency to run together again, and remain 

 in suspension in the water. The suspension of fine oil droplets, 

 which has the appearance of milk, is spoken of as an emulsion, 

 It can be at once destroyed by adding acid. This decomposes the 

 soap, setting free the fatty acids, which are insoluble in the water. 

 The pellicle around each globule is destroyed, and the globules run 

 together as neutral oil would in pure water. 



In order to characterise any given animal fat or mixture of fats the following 

 reactions are made use of : 



(1) The ' acid number ' of the fat, i.e. its content in free fatty acids, is deter- 

 mined by titrating it in ethyl alcohol solution with alcoholic solution of 



potash, using phenolphthalein as an indicator. 



(2) The ' saponification number.' This represents the number of milligrammes 

 of potassium hydrate necessary to saponify completely one gramme of fat. 



(3) The percentage of volatile fatty acids is determined by saponifying the 

 fat, then treating it with a mineral acid to set free the fatty acids and distilling 

 over the volatile acids in a current of steam. 



(4) The iodine number is the amount of iodine which is taken up by a given 

 weight of fat. It is a measure of the amount of unsaturated fatty acid present, 

 i.e. in ordinary fat, oleic acid. 



Besides the glycerides, a certain number of substances occur in the 

 body derived, not from a combination of fatty acids with glycerol, 

 but from a formation of esters of the fatty acids and other alcohols, 

 e.g. cholesterol or cetyl alcohol. Thus, spermaceti is a mixture of 

 cetyl palniitate with small quantities of other fats. The fatty secre- 



