40 



♦ KNOWLEDGE ♦ 



[Dec. 1, 1885. 



it cannot be formed by sulphur, phosphorus, metals, nor 

 by any other substance the phlogiston of which is not 

 in an oily state. Also, every oily matter treated with 

 fire in close vessels furnishes true coal ; so that whenever 

 a charry residuum is left we may be certain that the sub- 

 stance employed contained oil. Lastly, the inflammable 

 principle of coal, although it proceeds from oil, certainly 

 is not oil, biit pui'e phlogiston." (Modern scientists who 

 discuss dogmatically the properties of imaginary entities 

 should be warned by the fate of this poor old phlogiston, 

 or fire-ether, and study its philosophical analogy to the 

 luminiferous ether.) 



It is evident from this that, less than a century r,go, 

 the word " coal " was used among chemists in neai'ly the 

 same sense as we now use carbon and charcoal. Our 

 ordinary miner^,l coal was then distinguished r.s pit-coal, 

 or fossil-coal, just as the French .still describe it as 

 charbon de terre, the German as steinhohle, and the 

 Italians carbon fossile. We should remember that we 

 are an exceptional people in oiir daily and comnKm use 

 of fos.sil fuel. 



The prei^aration of wood charcoal is in other countries 

 one of the great national industries. Our colliery popula- 

 tion is replaced in Italy, for example, by the c?-rbonari, 

 and our " Black Country " by certain valleys into which 

 the trees growing on the mountain are floated down or 

 pitched down wooden slides, are peeled of their bark, 

 their trunks cut up for timber, and their branches made 

 into charcoal for domestic fuel. The methods of using 

 this, and the lessons we may learn therefrom, will be 

 treated when I deal with the sinful waste of coal that 

 here prevails, and its coming consequences. 



The fossil fuel used in this country may be divided 

 into thrae kinds, the ordinary bituminous coal, which 

 forms the bulk of the domestic fuel of England and 

 Scotland ; anthracite ; and cannel coal. Besides these 

 there are the graphite, referred to in my last, and which 

 I need not further describe, as it is not used as fuel 

 (excepting in the Bessemer converter), and Whitby jet, 

 which is reallj' a very compact and homogeneous cannel 

 coal used for ornaments. A fourth variety of fossil fuel 

 is used on the Continent, but is very little known in this 

 country. This is lignite, wood-coal, or brown coal. I 

 do not include the bituminous shales among coals, and 

 shall treat of them separately. 



If ordinary bituminous coal is distilled, i.e., placed 

 in a retort (which may be simply an iron tube or 

 iron bottle with an outlet tube), and this retort is 

 heated, there first rises through the outlet tube 

 vapour of water ; presently this vapour is accom- 

 panied with smoky vapour, which, when condensed 

 by cooling, is found to be a solution of salts of ammonia, 

 chiefly sulphate, of brown colour. The colour is due to 

 tarry matter. As the heating continues, and the tem- 

 perature rises towards a red heat, the vapour becomes 

 more smoky and inflammable. If the vapours are cooled, 

 we now have water, ammoniacal compounds, naphtha, and 

 tar, with more or less of non-condensible inflammable 

 gas, similar to that which we commonly burn. 



The order of their production is that in which they 

 are above named ; they are all given ofi all the while 

 under red heat ; but the first named continually decrease, 

 and the later named go on increasing as the temperature 

 rises. 



The proportions of naphtha, tar, and fixed gas vary 

 with the temperature of distillation ; the higher the 

 temperature the greater the quantity of gas and the 

 smaller that of the liquid naphtha. At a very high tempe- 

 ratiire the tar pjay be decomposed into gas and solid 



carbon. Therefore, in the commerci.il distillation of 

 coal, where the main product demanded is permanent gas, 

 as in oiu" ordinary gas-works, a very high temperature is 

 used. In distilling for liquid products the temperature is 

 moderated. To prevent rai.sunderstanding, I shovild add 

 that though the naphtha and tar are usually described 

 seiaarately, they come over in this first distillation com- 

 pletely mixed as crude tar — a treacly, combustible liquid, 

 which, by further distillation, is separable into the thick 

 pitchy matter commercially known as coal-tar, and the 

 limpid, liquid naphtha. By " fractional distillation" — i.e., 

 distilling step by .step at graduilly-rising temperatures — a 

 number of naphthas having different boiling-points may 

 be obtained. 



These volatile inflammable distillates all belong to the 

 chemical class of hydro-carbons, so named from their 

 composition, hydrogen and carbon. The hydro-carbons 

 of coal have also received the general name of bitumen, 

 from their resemblance, when crude, to mineral bitumen. 

 Hence the coal which contains them in notable ]>ropor- 

 tion is called bituminous coal. 



When these are driven off there remains behind in the 

 retort the well-known substance, gas-coke or " fixed 

 carbon." This, however, is not pure carbon, as is easily 

 proved by burning it carefully. Pure carbon burns away 

 entirely, combines with oxygen, and forms carbonic acid 

 gas. If the coke is burned, an incombu.stible portion 

 remains. This is mineral m;:tter varying in composition 

 with different varieties of coal. (The hard cirbonaceous 

 crust that lines the gas-retorts will be described here- 

 after.) 



By the simple processes above described we .separr.te the 

 coal into three chief constituents: volatile matter, fixed 

 carbon, and ash. It is customary to state the proportions 

 of these in describing commercially the composition of 

 different seams of coal. Regarded merely as fuel, this 

 information, added to the physical properties of hardness, 

 density, &c., are usually sufficient, but for some metal- 

 lurgical uses of coal and coke further jjarticulars concerning 

 the quantity of sulphur (of which there is always more 

 or less) and the constititents of the ash are necessary. 

 To the gas-maker and others engaged in the distillation 

 of coal a statement of the nitrogen contained in the coal 

 is important, as upon this depends the quantity of 

 ammonia he will obtain iu the form of saleable salt, 

 which is now an important bye-product. 



I shall have to return to this subject of composition of 

 coal again, but need not now go further into details, my 

 present object being to render intelligible the funda- 

 mental differences between the different classes of coal. 

 This is desirable, as I find that very few people have 

 anything like clear ideas on this subject. The difference 

 between ordinary bituminous coal, cannel coal, and 

 anthracite is a mystery to most Englishmen. 



Anthracite is easily understood. It is a natural coke. 

 It differs from both bituminous coal and cannel in the 

 absence of the volatile hydro-carbons that may be removed 

 artificially in the manner above described. I have given 

 some attention to the subject, and am quite satisfied with 

 the simple explanation which attributes its formation to 

 a natural distillation or roasting of one or the other of 

 these flaming coals. According to this, an anthracite 

 seam is a coal-seam that has been subjected to subter- 

 ranean heat, and simultaneously to the pressure of the 

 superincumbent rocks. We can produce artificial anthra- 

 cite by distilling coal and compressing it artificiallj-. 



Anthracite differs from artificial coke only in its 

 density or compactness, i.e., the absence of that porosity 

 which in artificial coke is due to the outflowing of the 



