366 



NATURE 



[January 



1 1 



H>12 



ail, tilt ......... ,, sluvv<d Uowii, and the gasct evolved are 



carbon dioxide und methane. 



It in rl'-ur that in a ma«>s of rottin{< vegetation under- 

 ({oin(f chocked decay, ft-rmcntation must play an im- 

 portant part ; and Renault found, in an extensive series 

 of rcxearches upon {K>at, that the most important factor 

 in the conversion of vptjctablc do|>osits intt» pf^al was 

 funf^i and bacterial ferments, which give rise to the pro- 

 duction of ulmic compounds of the composition : Carbon 

 65-3 'i hydrojien 3-85, oxygen 30-84. Mulder also, at an 

 earlier period, found that bodies could be extracted from 

 peat, to which he gave the name of humic and ulmic 

 acids ; and Kinof, Proust, and Hraconnot found that such 

 bodies formr^l the chief portion of peat. 



Thfsp humus bodies have also been frequ<'ntly identified 

 in the lignites and also in the true coals. 



None of these bo<lies are probably definite compounds, 

 and resemble the residues obtained by the action of dilute 

 acids on sugar and starch- The evidence, however, seems 

 to point to the presence in all bituminous forms of coal 

 of degradation products of the original vegetation of a 

 humus or ulmic character, and which is probably the 

 portion carrying the nitrogen ; and in round numbers 

 the proportions of the carbon, hydrogen, and oxygen will 

 be not far removed from : Carbon 62 per cent., hydrogen 

 5 per cent., oxygen 33 per cent. • 



It is also well known that tertiary coals, like the brown 

 coal and lignite depo>iits, are rich in fossil gums and 

 resins, derived from the extractive matter of the vegeta- 

 tion ; and a number of these have been isolated and 

 analysed ; whilst it is evident that in coal there are resin 

 bodies of this character approximating to the general 

 composition : Carbon, 79 per cent. ; hydrogen, 1 1 per cent. ; 

 oxygen, 10 per cent. 



The amount of resin constituents in the original vegeta- 

 tion, and which concentrates itself in the coal, must plav 

 an important part in chemical changes taking place 

 during the formation and ultimate composition of the 

 coal ; and it is clear that although the vegetation 

 that flourished in the coal age was of a verv different 

 character from that of later periods, yet in all proba- 

 bility the variations in the extractive matters of the 

 plants varied to much the same extent as in the flora of 

 to-day. Thus some deposits would be formed from vege- 

 tation containing but little of the resin-forming constituents, 

 while others would be rich in them. We know the wide 

 differences there are in the physical characteristics of the 

 lignites — sometimes more like wood than coal, at others 

 black, shining, and with a conchoidal fracture ; these 

 variations in appearance being due to the conditions under 

 which they have been formed and the amount of resin 

 constituents present. 



If we start with the humus and resin constituents as 

 they exist in the peat deposits of to-day, the latter ,nre 

 present only to the extent (t 5 m 10 per cent. ; hut in 

 the decaying vegetation of \\v . .u imniferous :i^. , tluv 

 were probably present in much larger quantities. The 

 humus, unprotected by it, rapidly undergoes decomposition, 

 with conceiitration of carbon and evolution of methane, 

 carbon dioxide, and water. As the layers of deposit .above 

 the carbonising mass {jrow thicker, so probably the tem- 

 perature rises. The ratio of resin constituents increasing 

 in proportion binds together the mass, and so helps to 

 protect the remaining humu-^ : .Tnd with the lapse of 

 centuries lignite is formed. If the amount of resin con- 

 stituents has been small, or, owingf to local circumstances, 

 has not been distributed evenly throughout the mass, the 

 lignite is loose in structure, and during the ensuing ages 

 continues decomposing until, if the pressure has been 

 great and the temperature high, nothing but the residual 

 basis and trace of resin constituent are left in the form 

 of steam coal or anthracite. Under other conditions they 

 may remain mixed with th.^ Mtumiiious coal in a seain 

 and form the "mother of (oal."' 



If the percentage of nsiii bodies has been verv high 

 — as in a drifted deposit of spores from lycopodia — and the 

 temperature has been high, the resin bodies may become 

 semi-liquid, and. minglinfj with surrounding earthv de- 

 posits, will give such compounds as boghead cannel. the 

 organic matter in which has the same composition as 

 resin, while it yields 33 per cent, of ash. Some of the 



NO. 2202, VOL. 88] 



canneis, however, are simply very ridi bituminou« '•":•' 

 When the temp«'rature has been high enough, - 

 the resin constituent* practically distil into the un'l 

 clay, yielding some forms of shale. 



Heat also may cause isomeric and other changes 

 resin bodies, thus altering thdr behaviour towards solv<-nts ; 

 while the effect of heat under pressure upon the resins 

 is in some cases to decompose them, with f..r..i-.»w>r. of 

 hydrocarbons, a long series of which wer< \<<j 



Kenard — among them being both saturated and ■ . d 



f roups, together with hydrocarbons containing < 

 iydrocarbons,' like retene (C,,H,,), have frequenil 

 isolated; and this body is found in many lignites, 

 the last few months, Pictet and Ramseyer have 

 hexahydrofluorene (C,,H,,) and others of the i..u: 

 aromatic hydrocarbons from coal — bodies which arc : 

 solved into aromatic hydrocarbons and hydrogen ■ 

 destructive distillation. Renard long ago isolated not o;; 

 saturated hydrocarbons like pentane and hexane, but n! 

 hexahydrides or naphthenes isomeric with the *■'■'■ 

 series, from the resin oil obtained by distilling wo<' 

 at a low temperature (350° C.) ; among these hexahyund 

 being C,U,,, C,H,,, and C,„H,,. The presence of bodi 

 of this character in low temperature coal tar is a furtlv 

 proof of the presence of the resin bodies in coal. 



All these degradation products of the original vegetati- 

 are to be found in the bituminous coals, the residual bo' 

 and humus forming the basis, which is luted together ' 

 the hydrocarbons and resins ; and the character' 

 various kinds of coal are dependent upon th< 

 in which the four groups of the conglomerate 

 These constituents of the coal have their own ( ' 

 products of decomposition when the coal is - 

 carbonisation. The humus bodies during carbonisati. 

 yield a large proportion of the gaseous products, and und 

 the influence of heat show no sign of melting, but 

 to break up at about 300° C. *The decomposition t. 

 more rapid as the temperature rises. Water disti! 

 in the early stages ; the tar is thin and poor in qi; 

 and the gases up to 600° C. consist of hydrogen, m- : 

 and carbon dioxide, with smaller quantities of carh. 

 monoxide and traces of other saturated hydrocarbor 

 The decomposition can be completed below 800° C. ; b 

 if the temperature is run up to 1000" C, the carb. 

 dioxide is reduced in quantity by the action on it of v. 

 red-hot carbon. Carbon monoxide increases correspor. 

 ingly, while hydrogen and methane are still evolved. 



The decomposition of the humus is also largely affect 

 by the rate of heating. If slowly heated, a large prop<. 

 tion of the oxygen is given off in combination with 

 hydrogen as water vapour, while if quickly raised in 

 temperature more combines with carbon to form carb. 

 dioxide and monoxide. The residue shows no sign 

 caking, while, like the naturally formed residue— mother 

 coal — it requires a large proportion of cementing mater 

 to make the particles cohere. The resin bodies and hydi 

 carbons which form the cementing portion in the ci. 

 melt between 300° C. and 320° C. ; and if a coars- 

 powdered sample of the coal becomes pasty or semi-fluid 

 at this temperature, it is a strong inference that the coal 

 will coke on carbonisation — a fact noted by Anderson, ap'^ 

 which is very useful in practice as a rough test. Ab<' 

 these temperatures, also, the resin bodies and hydrocarlxv 

 begin to decompose. 



The resin bodies at low temperature yield saturat- 

 hydrocarbons, unsaturated, chiefly hexahydrides or naph- 

 thenes, together with some oxygenated compounds : while 

 the hydrocarbons yield paraffins and liquid products — n" 

 these primary constituents undergoing further decomp*^- 

 tions at slightly higher temperatures. The liquids so pi 

 duced begin to distil out as tar vapours and hydrocarb. 

 gases, and leave behind with the residuum pitch, whi. 

 at 500° C. forms a mass already well coked together it 

 the residuum from the humus is not too large in quantitx . 

 The coke formed at this temperature is. however, soft ■ 

 but if the heat be raised to 1000° C. the pitch resid 

 undergoes further decomposition, yielding gas and leavi 

 carbon, which binds the mass into a hard coke. 



It has been shown by Muck and other observers that 

 is not always the coal containing the largest amount 

 volatile matter that evolves gas most rapidly or is rich' 



