Combustible Substances. 



volatile oils, and on examination show them- 

 selves to be natural compounds of the same ma- 

 terials. Some contain one-half their weight of 

 oil, and the matter left behind is resinous, which 

 may be restored to its former consistence by the 

 restitution of the oil. Many of the aromatic 

 oils, when distilled, leave behind them a resinous 

 substance ; and the resins, when first exuded, are 

 almost as thin as aromatic oils. 



Balsams and resins are usually obtained by 

 wounding the bark, &c. of the vegetables which 

 contain them, so as to let the juices flow out. 

 These juices at first are fluid, but upon exposure 

 to the air they become thicker by losipg some of 

 their water and more volatile parts. In some 

 few cases they are separated by spirit of wine. 



2. FIXED OILS are liquid, unctuous to the feel, 

 very combustible, of a mild taste, their boiling 

 point under 600. They are insoluble in water 

 and alcohol, and leave a greasy stain upon paper. 

 They are obtained partly from animals, and 

 partly from vegetables, and are often called ex- 

 pressed oils, from the manner of preparing or 

 obtaining them. They are produced and con- 

 tained in particular cells in considerable quan- 

 tities, so that mechanical pressure is often suffi- 

 cient for obtaining them. In some cases it is 

 done by heating the vegetable or animal matter, 

 and then exposing it to a strong pressure. It is 

 thus linseed oil is prepared. In other cases the 

 oily matter is boiled a considerable time in water, 



