220 BULLETIN 42, UNITED STATES NATIONAL MUSEUM. 



shales, sandstones, conglomerates, and occasionally limestone. It char- 

 acterizes and gives the name Carboniferous to a geological period, in the 

 rocks of which it occurs. Small amounts, however, are found in the 

 rocks of later periods. 



The varieties of coal are ordinarily classified by the increasing 

 amounts of volatile matter which are driven off" on being heated, but 

 the classes graduate into each other and sometimes overlap, since the 

 presence of more or less ash may change the per cent, of volatile mat- 

 ter from one class to another. As the volatile matter increases the fixed 

 carbon decreases. 



Anthracite contains up to 7 per cent, volatile matter. 



Semi-anthracite contains 7 to 10 per cent, volatile matter. 



Semi-bituminous contains 10 to 18 per cent, volatile matter. 



Bituminous contains over 18 per cent, volatile matter. 



While this classification has been in use many years and is conven- 

 ient, especially for trade purposes, a more rational and scientific classi- 

 fication is by the fuel ratio or the ratio existing between the fixed car- 

 bon and the volatile hydrocarbons, or the figure obtained by dividing 

 the per cent, of fixed carbon by the per cent, of volatile hydrocarbon. 



Q 



This is usually expressed . In this classification the water and 



V £1-0 



ash are regarded as impurities, and only the combustible portions of the 

 coal considered. Upon this basis the coals would be defined as follows : 



c 



VH-C 



Anthracite 100 to 12 



Semi-anthracite 12 to 8 



Semi-bitumiuous 8 to 5 



Bituminous , 5 to 



The bituminous coals are subdivided in a variety of ways, the prin- 

 cipal being coking coals, or those that form a coherent cellular coke on 

 being heated, noncoking, that do not form coke, canuel coal, very rich 

 in hydrocarbons, and brown coal or lignite, generally of more recent 

 formation and containing a larger proportion of oxygen. 



On being heated with free access of air all coals combine with the 

 oxygen of the air, or burn, with the generation of heat. This genera- 

 tion of heat on burning is the basis of the principal applications of 

 coal, and enormous amounts are consumed for this purpose. On being 

 heated in closed chambers or retorts some varieties of coal, particularly 

 caunel coal, give off large volumes of gas which can be collected and 

 subsequently used for illuminating and heating purposes, and leave a 

 residue of j)oor coke, which can be used for heating purposes. The 

 coking coals yield a hard strong coke on being thus heated, which is 

 very valuable for metallurgical purposes, but the gas evolved is gener- 

 ally allowed to go to waste. Large amounts are used for this purpsse. 



The production of coal in the United States in 1889 was 133,000,000 

 tons. 



