cm flie different Mnds of Coal 333 



in this, that coal ivS almost always a mixture of at least two dif- 

 ferent kinds, which are considered as a homogeneous whole ; 

 but, in this respect, coal does not form an exception to the ge- 

 neral law, that the chemical composition of an inorganic body is 

 manifested by its external properties. 



An intense black colour in coals, joined to a high degree of 

 lustre, as well as a considerable hardness, always indicate that 

 they contain a large quantity of carbon, and that the oxygen in 

 them predominates over the hydrogen. The species of lustre 

 determines the relation of the carbon to the other constituent parts. 

 Pitchy lustre indicates a smaller proportion of carbon ; the pas- 

 sage of this lustre to the semi-metallic indicates a greater. Black- 

 ness of colour, high lustre, and slight cohesion and hardness, cha- 

 racterise the coals which are rich in carbon, and in which the hy- 

 drogen predominates over the oxygen. A black colour, a dull 

 appearance, and a marked cohesion, with a certain degree of 

 hardness, are the signs which indicate a coal less rich in carbon, 

 in which the oxygen predominates in a high degree over the hy- 

 drogen. When the colour becomes a dark brown, it implies 

 that the proportion of hydrogen has increased with relation to 

 the oxygen. If, while the black becomes less intense, the coal 

 presents a duller aspect and an inferior degree of hardness, its 

 cohesion remaining the same, it is because the combustible still 

 contains less carbon, at the same time that the oxygen predo- 

 minates over the hydrogen. 



If it be wished, according to what has been stated, to deter- 

 mine precisely the nature of a coal, it appears sufficient to point 

 out whether or not the mass is homogeneous, and what are its 

 characters with respect to colour, lustre, cohesion, and hardness. 

 Should it be required, the carbonisation will make known the 

 quantity and appearance of the residuum in charcoal ; it will thus 

 complete the disclosure of the composition of the combustible. 



With regard to the specific gravity of coals, it presents few 

 means of characterising them, not only on account of acciden- 

 tal mixtures, but on account of all the variable circumstances 

 which may have accompanied their formation. In reality, coals 

 which are very rich in carbon commonly have a great specific 

 gravity, but this is only in the case where the oxygen predo- 

 dominates over the hydrogen. If the proportion of the latter 



