282 



NATURE 



[April 28, 192 1 



The Microstructure of Coal. 



A VALUABLE and original paper on the economic 

 selection of coal was contributed at the autumn 

 meeting of the Iron and Steel Institute by Mr. A. L. 

 Booth. The method usually adopted is to carry out a 

 proximate chemical analysis, which at the best is very 

 unsatisfactory and of little real use, to collate the 

 results with practical experience, and to make a trial 

 on some particular plant. Only too often it proves to 

 be unsatisfactory, and trouble arises from the fact that 

 two coals can have practically the same appearance 

 and give the same analysis, and yet be totally different 

 in behaviour. This occurs quite frequently, and does 

 not seem to be realised by fuel-users generally. 

 Sir W. G. Armstrong, Whitworth, and Co.'s works, 

 with which Mr. Booth is connected, use some 

 250,000 tons of coal per annum for different purposes, 

 and it was the unsatisfactory nature of chemical 

 methods of classification which led to experiments 

 being made with the microscope to ascertain whether 

 a more trustworthy method could not be devised. 

 The method adopted was as follows : 



Sections were cut of a large number of typical 

 pieces of coal from different sources. Some had been 

 proved over a period of years to be suited to a par- 

 ticular class of work, while others had proved un- 

 satisfactory for the same class of work. All were care- 

 fully examined under the microscope. It was soon seen 

 that there were three main types, and that each type 

 was suitable for certain classes of work. Further 

 investigation rendered it possible to decide how far 

 a departure from the typical member could be made 

 without getting into difficulties. 



The method of cutting sections is similar to that 

 used in making rock sections, but is considerably 

 more difficult and requires more patience. A piece of 

 coal is selected and, if soft and cracked, treated 

 with a transparent, colourless binder. One side of the 

 coal is then ground down, using carborundum powders 

 of finer and finer grades, finishing off with a water of 

 Ayr stone. The result should be a smooth, flat face. 

 The coal is then mounted in Canada balsam on a 

 piece of glass, the face being well pressed against it. 

 When the balsam is set, a slice of coal is cut off and 

 ground down until it transmits light. 



In his paper Mr. Booth considers only coals in com- 

 mercial use in this country, and these fall into three 

 main types : (i) " Humic," composed of leaves, stems, 

 and broken-down woody tissue, together with some 

 spores. (2) "Spore" coals, in which both "micro-" 

 and "mega-" spores predominate. (3) Cannel coals. 



The spores are the reproductive organs of the 

 plants, and correspond with the pollen and ovules in 

 present-<iay flowering plants. The micro-spores are 

 very small, while some of the mega-spores are about 

 \ in. in diameter. The cannel coals contain small, 

 round, yellow bodies. It will be realised, of course, 

 that these three classes merge into one another. Humic 

 coals occur containing more and more spores, while 

 spore coals become more cannellised as the yellow 

 bodies merge with the spores. This is where micro- 

 scopic work is necessary to enable a decision to be 



made as to what a particular sample of coal can be 

 used for. The author shows sixteen coloured photo- 

 micrographs of thin sections of specimens of the 

 three main types at magnifications varying from 

 50 to 560 diameters. 



So tar as the main economic uses of coal are con- 

 cerned, the study of their microscopic structure has 

 resulted in the following conclusions : 



For steam-raising, humic coals which contain a 

 fair proportion of spores are the most suitable. These 

 coals coke fairly well, and give a good, hot fire with- 

 out too long a flame. For town-gas manufacture 

 humic coals are also suitable, and for this purpose 

 those which swell on heating and burn with a long 

 flame are the best. They give a good yield of gas 

 and by-products. Some humic coals containing much 

 yellow substance constitute the best coking coals, and 

 should be reserved for that purpose. 



For producer-gas work the spore coals are neces- 

 sary. The best coals for non-recovery producers are 

 those which have been partially cannellised. They do 

 not soften, the coke is very fragile, and the fixed 

 carbon is very high. This is a necessary feature in 

 producer practice. If the ash is not very fusible it is 

 possible to work these coals with a low blast satura- 

 tion, and thus get a dry gas with a high carbon 

 monoxide content, the flame of which has a higher 

 radiating power than the hydrogen flame. In recovery 

 work, coal containing more humic matter may be 

 used, because here a primary low-temperature dis- 

 tillation takes place, and through the high saturation 

 of the blast the tendency to swell is checked. 



For direct-fired furnaces {e.g. reheating and rever- 

 beratory) the hard coals are used. These are almost 

 true cannels, and are usually dull-looking. They are 

 free-burning, having no tendency to coke, and unless 

 iron be present through infiltration it is difficult to 

 fuse the ash. 



The microscope has not only been found helpful in 

 the selection of coals, but in some cases it is also of use 

 in deciding whether or no it would pay to wash them, 

 and will explain why an apparently good and clean 

 coal has, for instance, a high ash-content. In such 

 a case a washing may be quite useless. In the event 

 of a shortage of a particular class of fuel the more 

 detailed knowledge of coal which the microscopic 

 study gives will enable the best substitutes to be used ; 

 and to obtain satisfactory working with the substitute, 

 any necessary alterations in the running of a plant can 

 be made without waiting for adverse effects to develop. 



The author states in conclusion that coal from the 

 same seam is generally very uniform, and mentions 

 that sections cut from a given seam, but delivered on 

 dates twelve years apart, showed that the coal is of 

 the same type. As he says, perhaps one day it will 

 be possible to buy coal to specification as we now buy 

 steel. 



Mr. Booth's paper is very timely, and indicates 

 what a considerable saving could be made if the 

 present output of coal were scientifically utilised in 

 the manner indicated. 



The Cretaceous-Tertiary Boundary in North America.^ 

 By Prof. A. C. Seward, F.R.S. 



ONE of the most difficult problems with which 

 American geologists and palaeontologists are 

 confronted is the correlation of the Later Cretaceous 

 and Lower Tertiary strata in the different regions of 



1 Department of the Interior, United States Geolo);icaI Survey. Profes" 

 sional Paper No. toi : "Geology and Palaeontology of the Raton Mesa and 

 other Regions in Colorado and New Mexico." By Willis T. Lee and F. H. 

 Knowlton. 



the United States. The Professional Paper by Messrs. 

 Lee and Knowlton is concerned with some of the 

 Cretaceous and Tertiary rocks in the Rocky Mountains 

 region of Colorado and New Mexico. A considerable 

 area in the interior of North America was occupied 

 by a Cretaceous sea, and it was part of this area 

 which was afterwards uplifted as the Rocky Moun- 



NO. 2687, '^OL. 107] 



