MICROSCOPIC STRUCTURE OF COAL. 753 



There are certain Coals, however, especially of the kind termed 

 'Cannel,' in which there is no distinct indication of Organic 

 structure ; although they present appearances so closely simulating 

 those of Vegetable tissue, as to have been mistaken for them by 

 experienced Microscopists. These Coals are made-up of an 

 aggregation of spherical or lenticular particles of a transparent 

 Bituminoid substance, imbedded in an opaque matrix of black 

 amorphous matter ; the predominance of the former or of the 

 latter constituting the difference between the ' brown ' and the 

 ' black ' Cannels.* 



587. In examining the structure of Coal, various methods may 

 be followed. Of those kinds which have sufficient tenacity, thin 

 sections may be made ; but the opacity of the substance requires 

 that. such sections should be ground extremely thin before they 

 become transparent ; and its friability renders this process one of 

 great difficulty. Any section must either cross the woody tissue 

 transversely, so that the appearance it presents will resemble that 

 of Fig. 215 ; or it must traverse it vertically, in which case the 

 fibrous structure will be brought into view, either as in Fig. 216, 

 or as in Fig. 217 ; or it must pass in an intermediate direction. 

 The following method, which would seem not only to be more 

 simple, but also to give more satisfactory results, is recommended 

 by the authors of the "iMicrographic Dictionary" (2nd Edit., p. 164): 

 ■ — "The Coal is macerated for about a week in a solution of car- 

 bonate of potass ; at the end of that time, it is possible to cut 

 tolerably-thin slices with a razor. These slices are then placed in 

 a watch-glass with strong nitric acid, covered, and gently heated; 

 they soon turn brownish, then yellow, when the process must be 

 arrested by dropping the whole into a saucer of cold water, or else 

 the coal would be dissolved. The slices thus treated appear of a 

 darkish amber-colour, very transparent, and exhibit the structure, 

 when existing, most clearly. The specimens are best preserved in 

 Glycerine, in cells ; we find that Spirit renders them opaque, and 

 even Canada balsam has the same effect." — When the Coal is so 

 friable that no sections can be made of it by either of these methods, 

 it may be ground to fine powder, and the particles may then, after 

 being mounted in Canada balsam, be subjected to Microscopic ex- 

 amination: the results which this method affords are by no means 

 satisfactory in themselves, but they will often enable the Organic 

 structure to be sufficiently determined, by the comparison of the ap- 

 pearances presented by such fragments with those which are more 



* This, in the opinion not only of the Author, but of his friends Dr. 

 Hooker and Mr. G. Busk who have examined the question in concert 

 with him, is the real nature of the much-vexed ' Torbane-hill Mineral ;' 

 which, though very different in structure and composition from the 

 ordinary 'bituminous' Coals, has exactly the same right to be termed a 

 Coal as have many ' cannels ' whose title to that appellation no one has 

 ever thought of disputing. 



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