90 ELEMENTARY AGRICULTURAL CHEMISTRY 



former — e.g., olive oil — are used for lubricating purposes, while 



the latter — e.g., linseed oil — are used in the manufacture of 



paints, linoleum, oil-cloth, varnishes. Oils and fats, both 



vegetable and animal, are also largely used in the manufacture 



of soap. 



Hard soap consists of sodium salts of various fatty acids, 



soft soap of corresponding potassium compounds. Soap is 



made by boiling the oil or fat with a solution of an alkali, 



whereby the metal of the alkali replaces the glyceryl of the 



oil, yielding the soap and glycerine (or glycerol, as it is now 



more systematically called). Thus, to take an example, if soda 



be boiled with glyceryl olcjate (the largest constituent of olive 



oil) the following reaction occurs : 



C3H,(C,,Il3.,COO)3 + SNaOH 

 Glyceryl oleate Sodium 



hydroxide 



= 3C3H,(OH)3 + 3NaC,,H33COO. 



Glycerol or Sodium oleate 



glyceryl hydroxide or soap 



Both the soap and the glycerine remain dissolved in the 

 water, from which the soap can be separated in the solid 

 state by adding common salt. Glycerine can be recovered 

 from the remaining brine. 



Oils may be present in various parts of a plant, but it is 

 always accumulated in the seed. Many seeds contain large 

 proportions of oil, sometimes up to half their weight. As a 

 rule, if a seed contains a high proportion of oil it is devoid of 

 starch, but many seeds — e.g., maize — which are rich in starch 

 contain a small quantity of oil. 



Oil is a concentrated source of energy, one part of oil being 

 equivalent to about two and a half parts of starch or sugar. 



Waxes are similar to the fats and oils in constitution, but 

 instead of the trivalent radical glyceryl, they contain mono- 

 valent radicals, of more complex character. 



III. Essential Oils and Resins. — Essential oils are 



usually volatile and possessed of characteristic odours. They 



^ear no resemblance chemically to the true oils. Many of 



