820 CHEMISTRY. 



fore, solid on the wood and in its pores. There is always 

 some creosote in the tar, and this preserves the wood from 

 decay or putrefaction. You see, then, the object of apply- 

 ing tar in the calking of ships. 



439. Coals found in the Earth. These are conveniently 

 divided into three classes: lignites, bituminous coals, and 

 anthracites. The first named has more nearly the compo- 

 sition of wood ; the second is an intermediate state ; and 

 the last, anthracite, is nearly pure carbon, having under the 

 combined influence of heat and pressure lost most of its 

 hydrogen and nearly all of its oxygen. 



Lignite is of a browner color than the others, and retains 

 in some degree its woody structure. Bituminous or soft 

 coal burns with a smoky flame containing some hydrocar- 

 bons, and hard coal burns with scarcely any flame at all. 



All three were made from woody substance, and were 

 brought into their present state by an imperfect combus- 

 tion. We see the same process essentially going on at the 

 present time, to a certain extent, in the formation of peat. 

 This substance is formed from marsh plants. There is a 

 growth of these every year, which, rotting in the water, 

 sink to the bottom. There occurs, therefore, in the course 

 of time, a large accumulation of vegetable substance, most- 

 ly woody fibre, in the form of a brown net-work, in which 

 the separate parts of the plants are discoverable. By 

 the partial decay that is, incomplete combustion of this 

 mass it is converted into peat, which is a half-formed coal, 

 being mostly carbon, having some oxygen and hydrogen 

 combined with it. 



The formation of coal in the earth will be particularly 

 noticed in Part III. 



440. Imperfect Combustion of Bituminous Coal. When 

 bituminous coal is heated with the air excluded, products 

 are obtained very similar to those which result from wood 



