494 THE CARBONIFEROUS SYSTEM. 



investigation of these structures, I may here notice very shortly the 

 methods by which my results have been obtained, more particularly 

 in the case of the mineral charcoal. 



In examining the mineral charcoal, I have, after many trials, adopted 

 the following process of preparation :— Specimens were selected con- 

 taining the tissues of only a single plant. Fragments or portions of 

 stems of this character can be obtained by careful manipulation from 

 most coals. They were placed in marked test-tubes, and treated with 

 strong nitric acid, in which they were heated to the boiling-point, 

 and kept in that condition so long as dense fumes of nitrous acid were 

 disengaged, or until, on looking through the tube, the material could 

 be seen to have a brown colour and a certain degree of transparency. 

 In many cases, boiling in this manner for a short time is sufficient to 

 render the fibres flexible, and as transparent as slices of recent wood 

 when slightly charred. When ready for examination, the charcoal 

 was allowed to settle, and repeatedly washed with pure water before 

 removing it from the tube. It was then examined in water, with 

 powers of from 50 to 300 diameters, drawings of the structures ob- 

 served being made with a camera ; and when it appeared desirable, 

 specimens were put up in balsam for further examination. Some 

 refractory specimens were found to require alternate washing and 

 boiling in hydrochloric and nitric acids before their structures could 

 be made out ; but in the preparation of more than four hundred speci- 

 mens from various kinds of coal I have scarcely met with any that 

 resisted all these processes.* 



I may observe here that the object is not to decarbonize the coal 

 and obtain what has been termed a siliceous skeleton. The change 

 effected consists in the removal of bituminous matter, which is oxi- 

 dized and dissolved by the acid, and of mineral matters, especially of 

 the sulphuret of iron, which is one of the principal causes of the 

 brittleness and opacity of the crude mineral charcoal. The prepared 

 material is nearly pure carbon, burning without flame and leaving 

 scarcely any ashes. It represents the cell-wall and its ligneous lining, 

 or perhaps in some cases only the latter, in a state of perfect integrity, 

 appearing under the highest powers quite smooth and continuous, 

 and with all its minute markings in excellent preservation. The 

 methods of incineration of the charcoal and of polishing its firmer 

 portions I have found to be, in comparison with the nitric acid process, 

 of little value. The first gives no adequate idea of the real character 

 of the tissues. The second gives merely a rude outline of the more 

 minute markings, and is chiefly valuable as affording cross-sections 



* This nitric acid process is, I believe, nearly the same with that recommended by 

 Goeppert and Morris. 



