EECEEATIVE SOIENCE. 



34f5 



of coal there are commercially, there are 

 only two kinds when microscopically re- 

 garded, viz., the opaque and the partially 

 transparent. When suitable sections of the 

 latter varieties of coal are made and reduced 

 to a uniform thinness, so that if transparent 

 they may be examined with transmitted light, 

 by the aid of powers which magnify from 

 seventy to three hundred diameters, they are 

 discovered to present certain general and 

 constant characteristic features connected 

 with vegetable origin and structure. They 

 are seen to be composed of three distinct 

 materials, which differ both in appearance 

 and in proportion. These are (1.) an opaque 

 black substance, which corresponds in micro- 

 scopical appearance and in reaction with 

 carbon as it exists in various natural sub- 

 stances. (2.) A yellowish and sometimes 

 reddish substance, the nature of which is 

 not easily ascertained. In colour it varies 

 from a light yellow to a bright red, or amber- 

 coloured resinous-looking matter. It enters 

 most abundantly into the composition of gas- 

 coals, while the black substance seems to 

 form the chief and almost exclusive con- 

 stituent of household coals and anthracites. 

 (3.) An earthy matter, more or less soluble 

 in water, which it sometimes colours. It 

 consists principally of umber. The relative 

 proportions of this matter seem to influence 

 the various products derived from the coal. 



All the varieties of coaly substances may 

 be classified under three heads, having re- 

 lation to what we have just stated : — 



I. Substances the sections of which are 

 for the most part opaque, and abound in 

 pure black matter. These comprehend the 

 household, coking, steam, and anthracitic 

 coals. 



II. Three varieties of coal, consisting of — 

 1. Coals more or less transparent, par- 

 ticularly gas-producing coals, having a clear 

 and conspicuous lustre, and a brittle, crystal- 

 line structure, such as the Pelton and Nova 

 Scotia kinds. 



2. Cannel-coals, like those of Wigan, in 

 Lancashire. 



3. Coals variously stratified, having an 

 earthy and black opaque coaly matter, such 

 as is known by the local term splint. 



III. Brown coals, lignites, Bovey coal, 

 and jets. 



These latter varieties are of far inferior 

 value as fuel, and therefore commercially 

 worthless where good bituminous coals are 

 available. Nevertheless they are most inte- 

 resting scientifically and under the! micro- 

 scope ; for we learn from sections of them that 

 the changes of the vegetable matter, which 

 have been completed in good Newcastle coal, 

 have evidently been suspended, or only im- 

 perfectly carried out, in the German brown 

 coal and the English Bovey coal. It may 

 be well, therefore, to explain what these sub- 

 stances are, and where they are found. 



Brown coal is so called from its brownish 

 colour, and is otherwise named lignite (from 

 the Latin lignum, wood) on account of its 

 woody appearance. It is indeed like half- 

 decayed wood ; though it is more compact in 

 some places where the vegetable remains 

 are so well preserved, and their original 

 structure so distinctly retained that a bo- 

 tanical indication is presented of the original 

 plants of which it was composed. These 

 consist of flattened stems crossing each other 

 in all directions, and are either of a darker 

 or lighter colour. They are soft and mel- 

 low in consistence when the substance is 

 freshly quarried, but become brittle by ex- 

 posure, the fracture following the direction 

 of the fibre of the wood. Of the same cha- 

 racter is the Bovey coal, so termed from 

 Bovey Tracy,' a village in Devonshire, where 

 it is found. It seems to be confined in our 

 land to that locality, but a very similar sub- 

 stance is found at Wettenau, in Germany, 

 and is styled fibrous brown coal. "We havo 

 in our cabinet a specimen of Bovey coal con- 

 taining minute seed-vessels of plants. 



All those varieties are sometimes called 



