Professor Harkness on Mineral Charcoal. 73 



differ amongst themselves. This will, therefore, be the same 

 as Sphene, so that the formula might be considered as 



fi Si § or (R 3 ft) Si S 



as the percentage of silica is the same as in Sphene. It ap- 

 pears to me doubtful whether we can consider any mineral 

 as true silico-titanates or silico-tantalates ; and it is probably 

 preferable not to do so, on account of the great differences in 

 properties between silicic and titanic acids. 



On Mineral Charcoal. By Robert Harkness, F.R.S.E., 

 F.G.S., Professor of Mineralogy and Geology, Queen's Col- 

 lege, Cork. 



Mineral Charcoal, or, as it is termed in some parts of Eng- 

 land, " Mother Coal," occurs in greater or less abundance in 

 almost every description of coal.* It usually presents itself 

 in the form of a black, pulverulent, fibrous, silky-looking sub- 

 stance, coating or embedded in the ordinary mass of the coal. 

 Sometimes, however, instead of having a fibrous structure, it 

 is somewhat granular, and both these forms may, in some 

 cases, be seen in the same coal. 



This substance, when fibrous, makes its appearance in a 

 shred-like state, but when it has a granular aspect it is fre- 

 quently manifest as a thin layer covering a face of the coal, 

 and these layers often form laminae among the seams of coal. 

 The occurrence of mineral charcoal in fossil fuel is not a cir- 

 cumstance which always prevails, and there are certain con- 

 ditions connected with coal-seams which lead to the preva- 

 lence, in some beds of coal, of this matter, while in others it 

 makes its appearance only to a very slight extent. 



On seeing a portion of mineral charcoal embedded in a mass 

 of ordinary coal, it will be at once perceived that it must have 

 owed its occurrence, in such a situation, to the influence of 

 causes which have not operated uniformly on coal-seams ; and 

 when we see a considerable mass of this substance associated 



* The substance here termed mineral charcoal is not, in its chemical compo- 

 sition, in all cases allied to anthracite, but is that matter which, in its external 

 aspect, somewhat resembles wood charcoal, and to which the name mineral 

 charcoal has been applied by mineralogists. 



