Combustible Substances. 223 



The use of the oil is to soften the acrimony of 

 the alkali, and give it a slipperiness, by which 

 more mechanical force can be employed in wash- 

 ing. 



To this class of combustibles we refer all the 

 fossil combustible substances, some of which are 

 fluid, others solid. The fluids differ in degree 

 of fluidity and subtilty. Among the principal 

 of these we reckon naphtha, asphaltum, amber, 

 jet, and pit-coal. The grand constituent of all 

 these is carbon, but so infinitely varied, that, 

 though the detail of their properties might be 

 entertaining, it would greatly exceed our limits. 



3. Tan is, however, a production of a different 

 nature; but its constituent principles have not 

 yet been ascertained. It differs according to the 

 vegetables from which it is produced. Nutgalls 

 are supposed to contain the largest proportion. 

 From its propensity to unite with the gelatinous 

 parts of animal matter, and to form with them a 

 hard substance, arises its great use in tanning 

 leather. It is also of extensive use in dyeing, 

 and in the making of ink. 



