372 CHEMISTRY. 



Oil-cloth is cotton cloth covered with colored varnish, and oil-silk is var- 

 nished silk. Drying oils are used in painting, and mixed with lampblack 

 they constitute printers' ink. 



521. Spontaneous Combustion. It is because of the rapid 

 absorption of oxygen from the air that the drying oils are 

 sometimes the cause of spontaneous combustion. This com- 

 monly occurs in waste thrown together in a heap. The 

 heat produced by the absorption of the oxygen sets fire to 

 the combustible substance cotton or linen or woolen that 

 is impregnated with the oil, and the oil, being itself com- 

 bustible, burns also. The reason that the heat is adequate 

 to produce this effect is that it is so shut in among the 

 parts of the heap where it is generated that it accumulates, 

 reaching at length the point of combustion. The oil in 

 drying always produces heat; for condensation of a gas, as 

 it combines with a fluid or solid substance, can not take 

 place without this effect ; but the heat in all ordinary cir- 

 cumstances quietly escapes into the air as fast as it is pro- 

 duced. In the drying of paint upon any surface heat is 

 formed at every point of it, but it produces no combustion 

 because it escapes instead of accumulating. 



522. Combustion of Fats. As both the fat acids and their 

 base, glyceryl, are compounds of carbon, oxygen, and hy- 

 drogen, we have in them the same elements as in wood 

 and coal, and therefore their combustion is attended with 

 the same phenomena and the same results. The facts stated 

 in Chapter X. fully illustrate this, and we need not dwell 

 upon the point here. 



523. Wax. This substance has so decided a resemblance 

 in some respects to the fats that it may be classed with 

 them. It is a mixture of two substances cerin and myri- 

 cin. A soap can be formed with cerin by boiling it in a 

 solution of potassium hydrate. Wax occurs in small quan- 

 tity in all plants. It gives a shining appearance to leaves, 



