SECTIONAL TRANSACTIONS.— G. 34f> 



debris, such as spores and pollens, so that the resulting coal may show local enrich- 

 ments of such bodies. 



Local contributions of different plant materials account for the commonly banded 

 appearance of bituminous coals. The differentiation of such banded coals into 

 physically distinct components — vitrain, clarain, durain and fusain — enables a study 

 to be made of the extent to which local enrichments affect the properties of coals, for 

 these components of banded bituminous coals differ from one another almost entirely 

 because of the different contributions made to them by the various parts of plants. 

 For proper understanding of the character of bituminous coal it is, in fact, essential 

 to study each physically distinct banded component separately. 



As a result of numerous researches on banded bituminous coals, it has become 

 increasingly apparent that the great complexity of character of the initial coal- 

 forming materials has not been transmitted to the resulting coal. During the 

 process of coalification and, as it would appear, during the initial stages, those of 

 decay of the vegetable matter, many of the more important components (in quantity) 

 of the accumulated plant-material lose their identity and by extensive alteration 

 and interaction find a common level as ' ulmin compounds.' These ulmins form a 

 definite class of compounds, not necessarily homogeneous, but probably comprising 

 several distinct types, yet sufficiently alike in their chemical constitution and behaviour 

 to justify their being grouped under one head. So far, no separation of the naturally 

 occurring ulmins into markedly different classes of compounds has been obtained 

 experimentally. 



The plant materials that contributed to the ulmin group of compounds are the 

 structural portions, the lignin and celluloses of the cellular framework, together with 

 much of the cell-contents, namely, carbohydrates and proteins. These materials 

 formed the bulk of the plants, and the resulting ulmins form the bulk of coal. The 

 predominance of ulmins is already apparent in an accumulation of plant debris after 

 a comparatively short process of decay, e.g. in peats, in which alkali-soluble ulmins 

 are found in increasing amounts as the age of the peat increases. The ulmins, as 

 first formed, are not permanent, unchangeable compounds, but are subject to pro- 

 gressive alteration according to the conditions to which they are subjected. Their 

 progressive alteration becomes most apparent in a decrease of solubility in alkalies, 

 partial in the lignites, complete in bituminous coals. Over the whole range of coals, 

 also, the ulmins progressively alter, more particularly as regards their ease of oxidation, 

 as the ' rank ' of the coal increases. There is good reason for the belief that the rank 

 of a coal is mainly determined by the change that has taken place (by such factors 

 as pressure, temperature and time) in the ulmins it contains. 



The parts of plants which do not undergo decay, with the formation of ulmins, 

 are the protective coverings of plant tissues, such as the coats or exines of spores and 

 the cuticles of stems and leaves, together with certain special plant products such as 

 the resins. These are not readily subject to decay, nor are they readily altered by the 

 conditions attending coalification, so that they are found in coal but slightly modified 

 from their original forms and in quantity greater than corresponds with their original 

 proportions in the plant debris. There are also present hi coal small quantities of 

 free hydrocarbons, probably derived from the oils and waxes of the plants during 

 the processes of decay and coalification. These three classes of plant materials, 

 protective tissues, resins and hydrocarbons, though different in character, can con- 

 veniently be grouped together as ' resistant plant remains.' 



A normal coal can be regarded as essentially a mixture of the two groups of 

 compounds, ' ulmins ' and ' resistant plant remains.' It can be confidently anticipated 

 that, as the result of work now in progress, the nature of any coal can be related to 

 (a) the character of its ulmin compounds, and (6) the contents and nature of its 

 resistant plant remains, so that a rational classification can be obtained. There seems 

 to be no possibility of compounding the influence of these two groups of materials, 

 for their effects on the character of the coal composed of them are independent. 



At present our knowledge of the extent to which each of these main component 

 groups of compounds affect the behaviour of coal under different conditions is 

 incomplete, but it is possible to relate one or other property broadly to the presence 

 of one or other component. For example, in those reactions of coal which involve 

 its oxidation the ulmin compounds play the major part ; while the behaviour of coal 

 on destructive distillation, as regards its yield of tar, is determined mainly by its 

 contents of resistant plant remains. The ' coking-power ' of a coal (its ability to 

 yield a commercial coke) depends on too many factors to permit of its being related 



