128 THE USES OF PLANTS. 



iii. Fats, 

 iv. Waxes. 



v. Volatile or essential oils. 



Of these, the first four groups, known as the Fixed 

 Oils, constitute a series of compounds formed, as was 

 shown by the classic researches of the centenarian, M. 

 Chevreul, between 1811 and 1823, of a glyceride, or 

 body analogous to the alcohol glycerin, C 3 H 5 (OH) 3 , in 

 combination with a fatty acid having, in nearly all 

 cases, the formula C n H 2n O 2 , or C n H 2n _ 2 O 2 . Linoleic 

 acid is C 16 H 28 O 2 , and ricinoleic, C 18 H 34 O 3 . The true 

 waxes contain no glyceride ; but some of those of 

 vegetable origin do so. If heated with steam under 

 pressure, the oil is decomposed into these two con- 

 stituents : thus Stearin, ( C H v>^ I O 3 , becomes 



^3^5 J 



Stearic acid, 3 c i8H^ 5 O j Q ^ and Glycerin if treated 



with alkalies, or basic oxides in the presence of water, 

 oils become saponified, the oxide uniting with the 

 acid to form a soap, i.e., generally a soda or potash 

 salt of the acid, glycerin being liberated. This is the 

 basis of the modern soap and candle industry, which, 

 since the removal of the duty on soap, in 1852, has 

 grown to an incredible extent. 



The essential oils are freely soluble in alcohol, or 

 in the fixed oils : they are not greasy, and at ordinary 

 temperatures they are nearly all limpid liquids. They 

 consist mainly of hydrocarbons, such as terpene, 

 C 10 H 16 ; but at a low temperature many of them 

 separate into a liquid ' elaeoptene ' and a solid ' stear- 

 optene.' Of these latter bodies, Camphor, C 10 H 16 O, 



