54 PHYSIOLOGICAL CHEMISTRY. 



high enough. Add more water and allow the whole to cool. A semi- 

 solid layer of fatty acids can now be lifted from the surface of the liquid. 

 The hardness of the mixed acids depends on the fat taken for experiment. 

 Mutton and beef tallows yield very solid acids ; with lard the mass is 

 softer, while with some oils the acids do not solidify at all at the ordi- 

 nary temperature. 



Ex. Dissolve a small portion of the fatty acids in warm alcohol, nearly 

 to saturation. On cooling, the acids separate in crystalline scales. 



Ex. The presence of glycerol as one of the products formed by the 

 saponification of fats is best shown as follows : 



Mix 50 cc. of cottonseed oil with 25 gm. of litharge and 100 cc. of 

 water in a porcelain dish. Place over a Bunsen burner on gauze and stir 

 until all oil globules have disappeared, adding a little water from time to 

 time. The litharge with water acts as lead hydroxide and saponifies the 

 fat, forming an insoluble lead soap, or plaster, and glycerol. When the 

 saponification is complete add more water, heat and stir well to dissolve 

 glycerol. Allow to settle a short time and pour the aqueous solution 

 through a filter. To the residue add water again, heat, allow to settle 

 and pour through the same filter. Concentrate the mixed filtrates to a 

 small volume and after cooling observe the sweet taste of the thickish 

 residue. 



Ex. Dissolve a portion of the sodium soap in water with aid of a little 

 alcohol. Then add some solution of calcium chloride or lead acetate. A 

 white precipitate is formed as the calcium and lead salts of the fatty acid 

 are not soluble in water. Hard waters, which contain salts of calcium 

 and magnesium, decompose soap in the same manner. 



Other Reactions. The common fats are insoluble in water 

 and when mixed with the latter tend to separate immediately. 

 However, it is possible to convert the fats and water into a 

 peculiar mixture called an emulsion which does not separate 

 into two layers on standing. In this condition the fat consists 

 of extremely minute globules which remain in suspension and 

 which may be passed through the pores of coarse filter paper. 

 It does not seem possible to secure an emulsion with perfectly 

 neutral fats, and in most cases the phenomenon depends on the 

 presence of a trace of soap formed. In the processes of diges- 

 tion of fats in the animal body emulsification plays a very 

 important part as will be shown later. It follows probably 

 the partial hydrolysis of the fats by lipase, referred to above. 



As they exist in the animal or vegetable organism the fats 

 are doubtless all amorphous substances, but in the separated 

 condition the solid fats always become more or less crystalline. 



