422- OMPOUND PRODUCTS. CHAP. I. 



emulsion. And on this account they are sometimes 

 denominated emulsive seeds. 

 Methods Some seeds yield their oil merely by means of 



of obtain- . . 1,1 



ing thepi. pressure, though it is often necessary to reduce tnern 

 first of all to a sort of pulp, by means of pounding 

 them in a mortar. Others require to be exposed to 

 the action of heat, which is applied to them by 

 means of pressure between warm plates of tin ; or 

 of the vapour of boiling water ; or of roasting be- 

 fore they are subjected to the press. 



But the oil which is thus expressed is still mixed 

 or combined with other substances, such as fecula, 

 starch, mucilage, which sometimes subside spontane* 

 pusly if the liquid is kept in a state of repose ; first 

 the grosser parts, such as the fragments of paren- 

 chyma that may have been expressed along with the 

 oil ; then the green fecula, then the starch, and 

 lastly the mucilage. The oil is now left in a state 

 of tqlerable purity, but not yet without a mixture 

 of other substances ; to divest it of which chemists 

 employ a variety of processes. 



Properties. Fixed oil, when pure, is generally a thick and 

 viscous fluid, of a mild or insipid taste, and without 

 smell. But it is never entirely without some colour, 

 which is for the most part green or yellow. Its 

 specific gravity is to water as Q*403 to 1 '000.* It 

 is insoluble in water. It is decomposed by the 

 acids, but with the alkalies it forms soap. When 

 exposed to the atmosphere it becomes inspissate^ 



* Connaiss, Chim. vol. vii. p. 325. 



