334 PRINCIPLES OF CHEMISTRY 



boilers, and in general for all purposes of heating and burning. 7 Russia 



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

 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 us we iind 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. a watery liquid, 

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

 charcoal incompletely burnt ; at 300 and higher a homogeneous mass like coal is formed, 

 which at 340 is dense and without cavities. At 400 the residue resembles anthracite. 

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

 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 medium quantity of ash is 5 p.c., but there are coals which 

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

 regards the amount of water, coal in this respect is much more profitable than lignite or 

 peat, as it does not usually contain more than 10 p.c. of water. The anthracites form 

 a remarkable variety of coals, and are indeed sometimes placed in a separate category ; 

 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 composition 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 the form of benzene or similar 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, 3 of hydrogen, and 3 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 most ancient forms of coal. The newest and 

 least tranformed, which resemble some of the brown varieties, are the dry coals (Griiner's 

 first group). 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 (2nd group, gas coal ; 

 3rd, smithy coal ; 4th, coking ; and 5th, anthracite, according to Griiner) in all respects 

 form connecting links between the dry coals and the anthracites. These coals burn with 

 a very smoky flame, and on being heated leave coke, which bears the same relation to 

 coal as charcoal does to wood. The quantity and quality of coke varies 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 par- 

 ticular the so-called bituminous coals are especially valuable, as even small coal (baked i 

 of this kind gives by 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 (55 p.c. of dense caking 

 coke. Such coal is very valuable for metallurgical purposes (see Note 8). Besides coke r 

 the dry distillation of coal produces gas (see further, illuminating gas), coal-tar (which 

 gives benzene, carbolic acid, naphthalene, tar for artificial asphalt, &c.), and also a 

 watery alkaline liquid (with wood and lignites the liquid is acid from acetic acid) which 

 contains ammonium carbonate (see Note 6). 



7 In England in 1850 the output of coal was already 48 million tons, and in latter 

 years (1884-1888) it rose to about 160 millions. Besides this the other countries con- 



