230 Br Charles Daubeny on the 



him we may collect, that quartz is nearly pure silex, coloured 

 by indefinite, though minute, quantities of certain foreign mat- 

 ters, but that mica and felspar are definite compounds, form- 

 ed of an acid and a base in exact atomic proportions. The 

 same, indeed, applies to all crystallized minerals consisting 

 of more than one ingredient, as was* first laid down by the 

 illustrious Berzelius — a generalization, by the by, which ranks, 

 perhaps, next to the great Daltonian law of definite propor- 

 tions, highest in the scale of importance amongst scientific 

 truths of this class, and was arrived at by him at a time when 

 so many difficulties lay in the way of its adoption, that we 

 cannot sufiiciently admire the penetration required for its dis- 

 covery, as well as the boldness which he evinced in promul- 

 gating it. Indeed, until the celebrated Prussian chemist Mits- 

 cherlich had developed his original views on the subject of 

 what he has called Isomorphism, we were stopped at the very 

 threshold of Berzelius' theory, by finding the same mineral to 

 contain, sometimes one base, sometimes another, without any 

 other apparent limitation, except that the proportions they 

 severally bore to each other should be as their atomic weights. 

 Thus garnet might contain alumina, peroxide of iron, lime, 

 magnesia, protoxide of iron, severally combined with silica, 

 or any one or more of these bases might be absent, provided 

 only there was one base with three atoms of oxygen com- 

 bined with two of the negative element, and another with 

 one atom of oxygen united with one of the latter, present in 

 the compound. 



The difficulty which this occasioned was, however, removed, 

 when Mitscherlich had shewn that several bases admitted 

 of being substituted one for the other without destroying the 

 essential character of the crystallization, or producing any 

 further change in it than a slight diff^erence in the angle ; 

 the only necessary condition being, that each of the bases so 

 replacing each other should contain an equal number of atoms 

 of oxygen. Thus potass, soda, lime, magnesia, protoxide of 

 iron, &c., may replace each other, as containing each an atom 

 of oxygen to an atom of the radical present ; as likewise may 

 alumina, peroxide of iron, and peroxide of manganese, since 

 they each contain three to two of base, so that the same mine- 



