CAKBON AND THE HYDROCARBONS 345 



now utilised everywhere as the principal material for heating steam 



component parts (peat and lignites, even in their last condition, still continue to evolve 

 nitrogen, carbonic anhydride, and marsh gases) form coed. Coal is a dense homogeneous 

 mass, black, with an oily or glassy lustre, or more rarely dull without any evident vege- 

 table structure ; this distinguishes it in appearance from the majority of lignites, the 

 density of coal (not counting the admixture of pyrites, &c.) varies from 1'25 (dry bitu- 

 minous coal) to 1*6 (anthracite, flameless), and even reaches 1'9 in the very dense variety 

 of coal found in the Olonetzky government (termed thungite), which according to the 

 investigations of Professor Inostrantzeff may be regarded as the extreme member of the 

 various forms of coal. 



In order to explain the formation of coal from vegetable matter, Cagniard de la Tour 

 enclosed pieces of dried wood in a tube and heated them to the boiling point of meroury, 

 when the wood was changed into a semi-liquid black mass from which a substance 

 exceedingly like coal separated. In this manner some kinds of wood formed coal which 

 on being heated left caking coke, others non-caking; precisely as we find with the 

 natural varieties of coal. Violette repeated these experiments with wood dried at 150, 

 and showed that when wood is decomposed in this way, a gas, an aqueous liquor, and a 

 residue are formed. The latter at a temperature of 200 has the properties of wood 

 charcoal incompletely burnt ; at 800 and higher a homogeneous mass like coal is formed 

 which at 840 is dense and without cavities. At 400 the residue resembles anthracite. 

 In nature probably the decomposition was in rare cases effected by heat alone ; more 

 generally it was effected by means of water and heat, but in either case the result ought 

 to be almost the same. 



The average composition of coal compiled from many analyses, disregarding the ash, 

 is as follows : 84 parts of carbon, 5 parts of hydrogen, 1 part of nitrogen, 8 parts of 

 oxygen, 2 of sulphur. The quantity of ash is on an average 5 p.c., but there are coals which 

 contain a larger quantity, and naturally they are not so advantageous for use as fuel. 

 The amount of water does not usually exceed more than 10 p.c. The anthracites form 

 a remarkable variety of coals, they do not give any volatile products, or but a very small 

 amount, as they contain but little hydrogen compared to oxygen. In the average com- 

 position of coal we saw that for 5 parts of hydrogen there were 8 parts of oxygen ; 

 therefore 4 parts by weight of the hydrogen are capable of forming hydrocarbons, because 

 1 part of hydrogen is necessary in order to form water with the 8 parts of oxygen. These 

 4 parts by -weight of hydrogen can convert 48 parts of carbon into volatile products, 

 because 1 part of hydrogen by weight in these substances combines with 12 parts of 

 carbon. The anthracites differ essentially from this : neglecting the ash, their average 

 composition is as follows : 94 parts of carbon, 8 of hydrogen, and 8 of oxygen and 

 nitrogen. According to the analyses of A. A. Voskresensky, the Grousheffsky anthracite* 

 (Don district) contains: C = 93'8, H = 1'7, ash = l'5. Therefore the anthracites contain 

 but little hydrogen capable of combining with the carbon to form hydrocarbons which 

 burn with a flame. Anthracites are the oldest forms of coal. The newest and least trans- 

 formed coals, which resemble some of the brown varieties, are the dry coals. They burn 

 with a flame like wood, and leave a coke having the appearance of lumps of coal, half ; 

 their component parts being absorbed by the flame (they contain much hydrogen and; 

 oxygen). The remaining varieties of coal (gas coal, smithy coal, coking, and anthracite), 

 according to Griiner in all respects form connecting links between the dry coals andj 

 the anthracites. These coals burn with a very smoky flame, and on being heated leave 

 coke, which bears the same relation to coal that charcoal does to wood. The quantity and 

 quality of coke vary considerably with the different sorts of coal from which it is 

 formed. In practice coals are most often distinguished by the properties and quantity! 

 of the coke which they give. In this particular the so-called bituminous coals are 

 especially valuable, as even the slack of this kind gives on dry distillation large spongy 

 masses of coke. If large pieces of these kinds of coal are subjected to dry distillation, 

 they, as it were, melt, flow together, and form caking masses of coke. The best coking 

 coals give 65 p.c. of dense caking coke. Such coal is very valuable for metallurgical . 



