V ON THE FORMATION OF COAL 141 



coal is made up of an accumulation of the larger 

 and of the smaller sacs. 



But, in one and the same slice, every transition 

 can be observed from this structure to that which 

 has been described as characteristic of ordinary 

 coal. The latter appears to rise out of the 

 former, by the breaking-up and increasing car- 

 bonization of the larger and the smaller sacs. 

 And, in the anthracitic coals, this process appears 

 to have gone to such a length, as to destroy the 

 original structure altogether, and to replace it by 

 a completely carbonized substance. 



Thus coal may be said, speaking broadly, to be 

 composed of two constituents : firstly, mineral 

 charcoal ; and, secondly, coal proper. The nature 

 of the mineral charcoal has long since been 

 determined. Its structure shows it to consist of 

 the remains of the stems and leaves of plants, 

 reduced a little more than their carbon. Again, 

 some of the coal is made up of the crushed and 

 flattened bark, or outer coat, of the stems of plants, 

 the inner wood of which has completely decayed 

 away. But what I may term the " saccular 

 matter " of the coal, which, either in its primary 

 or in its degraded form, constitutes by far the 

 greater part of all the bituminous coals I have 

 examined, is certainly not mineral charcoal ; nor 

 is its structure that of any stem or leaf. Hence 

 its real nature is, at first, by no means apparent, 

 and has been the subject of much discussion. 



