PLANT FOOD AND PLANT GROWTH 35 



series. All of the compounds here illustrated are known. The two 

 series differ only by the COOH group in the fatty acid series in 

 place of the CH 3 group in the hydrocarbon series. The CH 3 

 group is methane from which one hydrogen atom is removed, 

 leaving the radicle CH 3 , which is called methyl (one of the alkyl 

 radicles), and acts as a monovalent radicle, replacing one hydrogen 

 atom. It finds a place in many organic compounds, as in ethane 

 (CH 3 CH 3 ), butane (CH 3 CH 2 CH 2 CH 3 ) , etc. 



The group, COOH, is called carboxyl, or the acid group. It 

 may be represented: 



C O H 



II 

 O 



This group also has one free bond and acts as a monovalent 

 radicle. Whenever this group is contained in an organic compound, 

 the compound is an acid. The hydrogen in the hydroxyl part of 

 this group may be replaced by metals, thus forming salts. If the 

 free hand in this carboxyl group is grasped by a hydrogen atom, the 

 compound formed is formic acid, but if methyl ( CH 3 ) joins hands 

 with carboxyl ( COOH), the compound formed is acetic acid 

 (CH 3 COOH) , the acid which gives to vinegar its sour taste. When 

 lead (Pb) is used as a base to form a salt with acetic acid by re- 

 placing the acid hydrogen of the hydroxyl group, the sourness is 

 destroyed and the salt is known as sugar of lead, or lead acetate, 

 (CH 3 COO) 2 Pb. When the hydroxyl group joins alkyl radicles 

 ( CH 3 , C 2 H 5 , etc.), alcohols are formed, as methyl alcohol 

 (CH 3 OH), called wood alcohol, and ethyl alcohol (C 2 H 6 OH), 

 which is common alcohol. 



Common glycerin, which is also called glycerol (because it is 

 an alcohol), is an organic compound consisting of a trivalent 

 radicle, called glyceryl, united with three hydroxyl groups, 

 C 3 H 5 (OH) 3 . 



Common animal fats consist chiefly of palmitic, stearic, and oleic 

 acids combined with this radicle, =C 3 H 5 , and the fats themselves 

 are called palmitin, stearin, and olein. The harder fats, like tallow, 

 contain more stearin (C 17 H 35 COO) 3 C 3 H 5 , while the softer fats, like 

 lard and butter, contain considerable olein, which differs from 



