96 Agricultural Chemistry. 



produced by lower fatty acids. These acids rarely occur free, as 

 in the case of formic acid, which produces the sting of the nettle 

 plant; but they usually occur as constituents of neutral fats. 

 Oleic, linoleic and linolenic acids are types of three other series 

 of fatty acids which are more abundant in plants than the acetic 

 acid series. In distinction from the latter, these acids are char- 

 acterized by loose chemical bonds, by virtue of which their fats 

 take on oxygen, iodine and other active chemical elements. 

 Thus, on prolonged exposure to air, olein takes up one part of; 

 oxygen, linolein takes up two parts and linolenin takes up three 

 parts, by weight. This change is accompanied in proportion to 

 its extent by ' ' setting ' ' or hardening of the oils concerned. As 

 a result, while olein remains liquid even when exposed to the air 

 in thin layers and is characterized as a "non-drying" oil, in- 

 creasing proportions of linolein and linolenin produce con- 

 secutively the "semi-drying" and "drying" oils. 



The high percentages of the latter oils in linseed oil enhance 

 its value as a vehicle for paints, because, having distributed the 

 pigments which it carries, it gradually "sets" and forms a du- 

 rable protective coating. If the process of oxidation in such an 

 oil is hastened by exposing it in thin layers upon inflammable 

 material, sufficient heat may be generated to cause spontaneous 

 combustion. Ignorance of this fact has caused destructive fires, 

 due to oil soaked rags and similar material. 



Plant fats consist for the most part of mixtures of olein and 

 linolein with smaller amounts of stearin, palmitin and lower: 

 members of the acetic acid series. The proportions of fats ar 

 such as to maintain a liquid state at ordinary temperatures and" 

 produce the oils of the seed of cotton, castor bean, flax and other 

 plants. The simple fats differ from carbohydrates by a higher 

 content of carbon and hydrogen and lower oxygen content than 

 the latter. This higher content of combustible elements renders 

 fats of greater fuel value than the other leading plant compounds, 

 because of greater oxygen consumption during combustion. This 



