METHODS OF EXTRACTING OILS. 205 



Some seeds are valued chiefly for the oil which they yield, 

 e.g., rape, cotton-seed, linseed, castor, &c. There are three 

 chief methods employed in the extraction of oil : 



(1) Pressure ; the most generally employed. 



(2) Extraction by volatile solvents. 



(3) Boiling the bruised seed or fruit with water. 



(1) By pressure. By the first method the seeds are cleaned, 

 often deprived of their husks or shells ("decorticated"), 

 crushed, and ground, often heated and moistened, and then 

 subjected to intense pressure in hydraulic presses. The 

 crushed seeds are placed either in horsehair bags or in moulds 

 and are pressed, first at the ordinary temperature, whereby 

 "cold drawn oil" is obtained, and then after heating " hot 

 drawn oil " is extracted. The oil obtained is refined by 

 heating it to a temperature of about 160 C., in order to coagu- 

 late albuminoid matter, and sometimes by treatment with a 

 little sulphuric acid. The residue left in the press constitutes 

 " oil-cake" and usually contains from 6 to 15% of oil. 



It is used for feeding or manurial purposes, since in addition 

 to the oil it contains the albuminoid and mineral matter 

 present in the seed. 



(2) In extraction by solvents the crushed seeds (in bags or 

 cages) are treated with solvents, such as petroleum naphtha 

 or carbon disulphide, in such a way that the fresh seeds are 

 treated first with the solvent already highly charged with oil, 

 while the fresh solvent comes first into contact with the seeds, 

 w r hich are almost deprived of oil. In this way a much more 

 complete exhaustion of the seed is obtained, leaving a residue 

 almost devoid of oil, and therefore of much less value as a 

 feeding stuff than the usual oil-cake. 



(3) Extraction by boiling with water is sometimes used in 

 the extraction of castor oil (mainly by the natives of the coun- 

 tries where the seed is grown) and of olive oil. The oil in 

 such cases is merely skimmed off the top of the water. 



The Waxes are substances probably occurring in small 

 quantities in many plants. They are generally analogous to 

 the fats in composition, except that instead of the trivalent 

 glyceryl group they contain monovalent groups ; they therefore 



