82 PHYSIOLOGICAL CHEMISTRY 



its melting point, often to such an extent that the fat is liquid 

 at ordinary room temperature. Such fats are called oils. 

 Other unsaturated acids are found in some fats, or "oils," and 

 they exert a similar influence. 



Among these unsaturated acids are linolic (C 18 H 32 2 ) and 

 linolenic (C 18 H 30 2 ). Oils which contain considerable amounts 

 of the esters of these acids undergo oxidation easily and are 

 converted into a hard resinous material when exposed to air 

 and light. They are called drying oils, and are used in paints 

 and varnishes. Linseed oil is an example of this group. 

 Others, such as cottonseed oil only thicken on exposure to air 

 and light; they are called semidrying oils. Olive oil does not 

 harden under similar circumstances and is classed as a nondry- 

 ing oil. 



General Properties. The solid fats are white or light yellow 

 substances, which if pure are odorless and tasteless. The oils 

 often are yellow and frequently have a decided taste and odor. 

 They are insoluble in water, somewhat soluble in cold alcohol 

 but much more so in hot alcohol, and soluble in ether, chloro- 

 form, benzol, etc. From solutions, the fats often may be ob- 

 tained in crystalline form as long needles. The specific grav- 

 ities of all the fats and oils are less than that of water, hence 

 they float at the surface. They reduce the surface tension of 

 water. The naturally occurring fats and oils do not have sharp 

 melting points, since they are mixtures of different kinds of 

 fats. Even pure fats often show much indefiniteness in -melting 

 point. Some melt, resolidify at a slightly higher temperature, 

 and if further warmed, melt again. This is supposed to be due 

 to the fact that an internal rearrangement in the molecule is 

 brought about by heating. 



Emulsification. If neutral oil and water are shaken to- 

 gether vigorously, and the mixture allowed to stand, it will 

 quickly separate into two layers, oil and water. If a small 

 amount of soap solution is added to this mixture and the shak- 

 ing repeated, the liquid becomes milky in appearance, and even 

 after prolonged standing will fail to separate into two layers. 



