OILS. 203 



quence of the strong affinity "which lime has for oxalic 

 acid, and the fact that the compound which they 

 form, is insoluble in water, oxalic acid is a very valua- 

 ble test for the presence of lime and all its soluble 

 compounds. 



505. The super-oxalate of potash is a soluble 

 crystalline salt; it is found in sorrel, rhubarb, and 

 many similar plants, giving them a sour flavor; it 

 occurs in many fruits associated with malic and tar- 

 taric acid. This salt, which is termed salts of sorrel, 

 and sometimes also (though most improperly) salt of 

 Jemons, is used in taking out iron-moulds, or stains 

 in linen caused by oxide of iron. This use depends 

 upon the fact, that the oxalic acid has a strong affinity 

 for oxide of iron, and forms with it a readily-soluble 

 salt, whilst the acid used has not corrosive powers, 

 and therefore does not in any way injure the texture 

 of the linen. 



506. The oily substances formed by plants are like- 

 wise very numerous; they are of two kinds, fixed and 

 volatile; those which when exposed to the air remain 

 without diminishing their bulk, and those which when 

 so exposed evaporate. Olive oil, castor-oil, and 

 cocoa-nut oil, belong to the former class, whilst oil 

 of lavender, oil of cloves, and oil of lemons, are vola- 

 tile oils. 



507. All vegetable oils, whether fixed or volatile, 

 contain carbon and hydrogen; in addition to which 

 the greater number contain a portion of oxygen. 

 They are all combustible, and when burning form 



