January ii, 19 12] 



NATURE 



;67 



in hydrocarbons, and this naturally follows from the fact 

 that the coals which have the highest oxygen percentage 

 are mostly those giving high volatile matter. As these 

 are rich in the humus bodies which yield most of the 

 diluting gas and but little tar or rich hydrocarbon gases, 

 they cannot give the high result of a coal in which the 

 oxygen content is about lo per cent, or rather lower, and 

 which contains a large percentage of resin bodies. 



Experience shows that the weathering of coal is a 

 phenomenon which is dependent upon the absorption of 

 oxygen from the air ; and this weathering is fatal to the 

 coking of some coals, the slacks of which are so susceptible 

 to oxidation that a few days' or weeks' exposure destroys 

 their coking power. Now the avidity of oxygen for some 

 Vegetable resins is well known ; the rapidity with which 

 iiipal will absorb oxygen from the air may be taken as 

 example. Common resin has itself been formed by the 

 idation of turpentine, and countless ages under condi- 

 ins tending to reduction may well have whetted anew 

 ' resinic appetite for oxygen. In any case, the resin 

 >dies are the compound-; present in the coal most likely 

 possess this property ; and it is the chemical actions so 

 used which lead to slow combustion, and, when 

 t;elerated by any rise in the surrounding temperature, is 

 pable of generating suf!icient heat to lead to the spon- 

 neous ignition of masses of broken coal large enough to 

 ■vent the escape of the heat as it is developed, 

 ('oal exhibits, to a lesser extent, the same property of 

 '-.orbing gases that charcoal does. The least absorbent will 

 k-e up one and a quarter times its own volume of oxvgen, 

 iiile many bituminous coals will absorb more than three 

 nes their volume of the gas. This action, at first largely 

 vsical, presents the oxygen in a probably active condi- 

 gn to the resin bodies in the coal, and leads to the rapid 

 "weathering" and destruction of the coking properties 

 found with some kinds of coal. 



Boudouard has shown that when coal is weathered 

 humus bodies are produced, and the coking power is 

 lessened or destroyed. In seven samples of various coals 

 the humus constituents were increased by the oxidation, 

 uhich seems to show that the action of the absorbed 

 oxygen is to attack the resin compounds ; and as we know 

 that carbon dioxide and moisture are the chief products of 

 the earlier stages of heating of masses of coal, it seems 

 probable that the result is a conversion of resinic into 

 humus bodies with evolution of these gases. It is this 

 ( hange that leads to the serious deterioration in the gas 

 and tar made from coal which has been too long in store ; 

 while the fact that a cannel coal like boghead or a shale 

 does not weather is partly due to its dense structure, and 

 also, in the same way, is an indication that the resin 

 bodies of which it is chiefly composed are of a different 

 Ivpe — a fact borne out by their resistance to certain coal 

 K-ents which freely attack the ordinary resin matter. 

 It has been shown that the coals richest in resin bodies 

 the cannels, whilst those that contain most of the 

 -idues of the humus bodies and least of the resin con- 

 stituents are the steam coal and anthracite, and between 

 th'se extremes come the large class of bituminous coals. 

 Many classifications of coal have been suggested, some 

 -'^d on their chemical, some on their physical, and others 

 their coking properties. Of the latter, the most gener- 

 ally adopted is that suggested by Gruner, in which he 

 tabulates bituminous coals into five classes. .\lthough 

 Schondorff, Muck, and others have shown that it is not 

 applicable to all kinds of coal, still this criticism applies 

 to all classifications that have been proposed. 



Carbon Hydrogen Oxygen 



n,., (-«»! /Lone flame and"! . « . . . o. 



Dry Coal ... -j „„„.,„;,i,g .J- 75-8o 4-3-5-S la'o-iSs 



Fat gas coal ...{C'jl;!^^^^""^ ='"''} 80-85 S^o-S'S io-o-i3-2 



/Good coke, but\ «. q.., ,.- .. 



\ porous... .../ ^♦-^^ 50-55 5-5-'o-o 



Semi-fat or ftir 



nace coa 

 Coking coal ... Best coke ... 89-91 



Lean coals and 1 xr , • 



anthracite ... | Non-coking ... 90-93 



4'S-5"S 

 3'o-4'3 



4'5- S'S 

 3'o- 4'5 



I his arrangement shows not only the coking properties, 

 also the changes in composition which the coal imder- 

 ^, the concentration of carbon, and reduction in highly 

 Used bodies. In the first class we have the dry coals, 

 Iding large volumes of gas and liquid products on dis- 

 tillation ; and these — as might be expected — most resemble 



NO. 2202, VOL. 88] 



the lignites, and share with them the property of non- 

 cokmg or binding together of the residue on carbonisation. 

 This IS due to the fact that the humus-like bodies are still 

 present in much larger quantities than the resinic com- 

 pounds and hydrocarbons, and as on distillation they leave 

 no bmding material in the residue, the resinic bodies can- 

 not supply enough to give more than a friable mass. 



In the second class A coals, altered conditions of 

 temperature, pressure, and time have led to further de- 

 compositions of the humus bodies, and the resinic con- 

 stituents and hydrocarbons having increased in ratio by 

 concentration, a point is reached at which coking take's 

 place, although not of a really satisfactory character. 



In the third class, the action has still continued with 

 further concentration of the resin bodies, hydrocarbons, and 

 residuum, with the result that the former bodies are so 

 increased in comparison to the humus and residuum that 

 a good coke results, although, for reasons that will be 

 discussed when speaking of coking processes, it is rather 

 too porous and bulky. 



In the fourth class, the proportion of resin and hydro- 

 carbon bodies has reached the right ratio as compared 

 with the humus and residuum, and the best coking coal 

 is obtained. Bituminous coals of the kind classified bv 

 Gruner may therefore be looked upon as an agglomerate 

 of humus and the degradation products of these bodies 

 down to carbon, luted and protected by resin bodies and 

 their derivatives ; steam coal and anthracite as the degra- 

 dation products of humus which has nearly completed its 

 decomposition owing to the small quantity of resin bodies 

 in the original vegetation ; cannel coal as consisting 

 mainly of resin bodies, which, having been in a semi-fluid 

 condition, have mingled with the earthy matter in contact 

 with it, so obtaining the high ash found in many kinds. 



In putting forward this theory as to the composition of 

 coal, I wish it distinctly understood that by the terms 

 humus " or " resin " bodies I do not imply any one 

 definite compound, but merely bodies of this character — 

 the humus bodies all containing a percentage of hydrogen 

 from 5 per cent, downwards, while the resin bodies all 

 contain a percentage of hydrogen above 5 per cent. If it 

 is once admitted that coal is a conglomerate of the kind 

 I have indicated, it explains all those obscure points which 

 no other theory touches — such as why with two coals of 

 almost identical composition and of high oxygen content 

 one should be a coking and the other a non-coking coal, 

 the reason being that in the one the high oxygen content is 

 due to humus bodies, which will not coke owing to the 

 low pitch-forming nature of the hydrocarbons, while with 

 the other the oxygen is due to resin bodies, which are 

 essential to good coking. 



In 1898 Anderson and Roberts, as the result of a long 

 research upon the chemical properties of Scotch coals, 

 came to the conclusion that a considerable part of the 

 organic matter in coal consists of a complex compound 

 comparatively rich in nitrogen, and also containing sulphur, 

 and that there Is also present resinous material, while the 

 remaining constituents are composed of degradation pro- 

 ducts of the original carbohydrates of the coal plants, a 

 theory which in its essentials agrees very well with my 

 views on the subject. 



During the present year (191 1) Burgess and Wheeler 

 have published the results of a series of experiments upon 

 the distillation of coals at various temperatures which lead 

 them to conclude that coal contains two types of com- 

 pounds of different degrees of ease of decomposition. The 

 more unstable decomposes below 750° C, and yields on 

 distillation the paraffin hydrocarbons and no hydrogerv ; the 

 other decomposes only at or above 750° C, and yields 

 hydrogen only, or possibly hydrogen and oxides of carbon. 

 The latter they suppose to be a degradation product of 

 cellulose ; the former to be derived from the resins and 

 gums from the coal plants. The authors consider that the 

 difference between one coal and another is determined by 

 the proportion in which these two types exist in the coal. 



.AH the evidence that can be adduced shows that when a 

 coal undergoes destructive distillation all the hydrocarbons, 

 together with the resin and humus constituents, undergo 

 decomposition at a temperature certainly well below 

 700' C, and that as the liquid and gaseous products distil 

 out they leave behind their less volatile residues as a pitch, 

 which lutes together the carbon particles and forms soft 



