FATS, WAXES, AND LIPOIDS 151 



" unsatisfied' ' valencies may be illustrated by the formulae of 

 ethane and ethylene : 



H H H H 



C=C H— C— C— H 



H H H H 



ethylene ethane 



In the ethylene it is seen that there are not enough hydrogen 

 atoms to satisfy all the valencies, and in order that none may be 

 left unsatisfied the remaining valencies must be joined to each 

 other, thus uniting two carbon atoms by more than a single bond. 

 Such compounds are said to be unsaturated, that is, they can easily 

 take on more atoms before their full quota would be used up, ac- 

 cording to the valencies of the atoms present. 



The above fatty acids are all saturated and belong to the acetic 

 or stearic acid series. Other series, such as the oleic and linoleic 

 series, are unsaturated. In the oleic acid series are oleic acid 

 (C17H33.COOH) from olive oil, and brassic acid (C21H41.COOH), 

 which occurs in rape and mustard oil. The general formula is 

 CnH 2n -i.COOH, and they are thus seen to be unsaturated. 



The linoleic acid series has the general formula CnH 2n -3.COOH 

 and is represented by linoleic acid (C17H31.COOH) found in lin- 

 seed oil. The explanation of the drying properties of this oil is 

 now obvious. Since it is so far from being saturated, it takes on 

 oxygen readily when exposed to the air and increases in weight, 

 at the same time becoming stiff and hard. For this reason those 

 fats containing a high percentage of unsaturated acids are called 

 " drying" oils and are used largely in paints, varnishes, etc., 

 while those which are more nearly saturated are nondrying and 

 are used more for food and lubricating purposes. 



As previously mentioned, the fatty acids all contain an even 

 number of carbon atoms. About 80% of the acids of the glycer- 

 ides which make up the fats of plants and animals are those such 

 as oleic, linoleic, ricinoleic, and stearic acids, all of which contain 

 18 carbon atoms. This is a multiple of six and must bear some 

 relation to the fact that the principal starches and sugars also 

 contain multiples of six carbon atoms in their molecules. In any 

 discussion of the origin of the fats and their relation to the carbo- 

 hydrates, this should be borne in mind. 



