LIVING SUBSTANCE 13 



glycerine. The acids, with few exceptions, belong 

 to the " fatty-acid " series. Three molecules of 

 fatty acid, not necessarily of the same kind, combine 

 with one of glycerine to form a fat. The cleavage 

 and recombination of these two component parts of 

 a fat (the fatty-acid element and the glycerine) is 



FIG. 2. Milk under the microscope, showing the nature of an 

 emulsion. The spheres are fat globules, the dark rods lactic-acid- 

 forming bacteria. (From " Elements of Biology," copyright, 1907, by 

 George William Hunter. Permission of the American Book Co., 

 publishers.) 



easily accomplished. When the fatty-acid portion 

 combines with an alkali, it forms a soap. Fats are 

 insoluble in water, but readily soluble in ether, ben- 

 zine, etc., a fact made use of in the cleaning of 

 clothing. They may be shaken up in alkaline water 

 until the particles become very finely divided and 

 remain in suspension, forming an emulsion. The 

 most familiar example of such an emulsion is milk. 



