62 M. Karsten'*s Observations and Experiments on 



tioiis in the ashes of coals are in general silica, alumina, lime and 

 magnesia. The two latter commonly occur in much smaller 

 quantity than the others. 



The author shews by examples, that the determination of the 

 contents in ashes of a coal, even in specimens selected as the 

 purest in a bed of this combustible, cannot, on account of the 

 infinite variety of local circumstances and of position, furnish 

 data which could afford any conclusion as to the whole deposit ; 

 but convinced, by more than two hundred and fifty trials of 

 coal, that there exists an essential difference in the contents in 

 earths, of coals of different natural deposits, he admits that the 

 quantity which represents the contents in ashes, especially if it 

 be determined by the examination of several specimens of the 

 same coal, may always be considered as a mean term which ap- 

 proaches the truth. 



In respect to this determination, it is of importance to se- 

 parate from the coal the foreign substances which sometimes 

 occur in its natural fissures. These substances are in gene- 

 ral iron pyrites and calcareous spar, sometimes dolomite, galena, 

 blende, sulphate of barytes, carbonate of iron, selenite and 

 siliceous clay. In consequence of this precaution which the au- 

 thor recommends, and because also the trials of coal on a small 

 scale can only be confined to the pure mass, it is evident, that 

 the contents in ashes will always be much less in the small than 

 they would be in the great. M. Karsten concludes from this, 

 that, in order to be enabled to judge of a coal with reference to 

 its employment in the arts, it is not sufficient to know^ the quan- 

 tity as well as the circumstances of the coke which it furnishes, 

 and its contents in ashes, but it is also necessary to indicate in 

 detail the usual state of the fissures of the coal, and to mark if 

 the surfaces of these fissures are clean, or if they are filled with 

 foreign substances. 



It is in fact in this manner that the author has proceeded in 

 his numerous trials. 



After these general remarks and many others, which we are 

 obliged to pass over in silence, M. Karsten describes the appara- 

 tus which he employed for his eleven analyses in the dry way, 

 analyses which, as we have already seen, were afterwards to 

 guide him in his researches. It is that which has been employed 



