THE COMPONENTS OF PLANTS AND ANIMALS 19 

 MELTING POINTS 



Stearic acid 71.5 C. 



A distinction is commonly made between fats and oils, the 

 fats being solid at ordinary temperatures and the oils liquid. 

 The difference depends largely upon the proportion in which 

 the various simple fats are present. Olein and other fats con- 

 taining unsaturated acids are usually liquid at room temper- 

 ature and their presence increases the softness of the fat. 



The fatty acids of higher molecular weight are volatile only 

 at comparatively high temperatures and at reduced pressure. 

 Those of lower molecular weight, notably those contained in but- 

 ter fat, can be readily distilled in a current of steam and their 

 proportion serves to distinguish butter fat from other animal 

 fats. 



An important physical property of the fats, which, however, 

 is by no means peculiar to them, is that of forming what is 

 known as an emulsion. Fat is said to be emulsified when, in 

 the liquid state, it is distributed in minute droplets or globules 

 throughout some other liquid ; for example, if fat be violently 

 shaken with water an emulsion is formed. Such an emulsion 

 is not permanent, however, the fat droplets very soon coalescing 

 and rising to the surface. The presence of small amounts of 

 certain other substances dissolved in the water, however, will 

 prevent this separation and give rise to a permanent emulsion. 

 The most common substance producing this effect is soap. 



Certain gums and also proteins likewise serve to retain fat 

 in the emulsified state. The most familiar example of such an 

 emulsion is milk, the fat being held in suspension in this case 

 by the action of the proteins of the mUk. This effect of various 

 substances in retaining fat in the emulsified form depends upon 

 their effect upon the surface tension of the contact layer be- 

 tween fat and water, but a full discussion of this point would 

 be out of place in this connection. 



33. Native fats. As has already been stated, the reserve 

 fats of the animal body are triglycerids, chiefly of stearic, oleic 



