840 Professor Necker on Mineralogy considered 



the primitive, will be reputed to belong to the same species, be- 

 cause the law which indicates their composition may be clearly 

 designed by a chemical or mineralogical formula, and their form 

 by a more or less complicated, but always precise crystallogra- 

 phical symbol or definition, being itself a law, or an assemblage 

 of crystallographical laws. 



§ 50. In such a case, the greater or lesser extension assumed 

 in different crystals of the same form, by their limiting planes 

 and the geometrical form of these planes being determined by 

 circumstances apparently so accidental and so particular as are 

 not capable of being brought under the expression of any gene- 

 ral law, will be considered by us as mere varieties; differences 

 of colours in individuals of the same species, when they are 

 produced by the presence of some ingredient in too small a 

 proportion to be included in the chemical formula, will also 

 rank as varieties. 



§ 51. Thus, according to our views, the primitive or cubic, 

 the octahedral, and the triform, galena or sulphuret of lead, 

 will be three distinct species of crystals, while the three different 

 aspects which this last species, the triform, may assume, viz. a 

 cube or an cxjtahedron with truncated edges and angles, and a 

 rhomboidal dodecahedron with all its angles truncated, will be 

 only varieties of the same species, being merely accidental and 

 unimportant modifications of a general form, otherwise deter- 

 mined by precise laws. 



Likewise the regular six-sided prisms and the six-sided pyra- 

 mids of corundum will be two distinct species of crystals ; but 

 the blue or red colour which such crystals may exhibit, will only 

 characterize a variety, for the quantity of oxide of iron, by which 

 this colour is produced, is too small to appear in the formula 

 Al or Al, which denotes the chemical composition of the corun- 

 dum. 



§ 52. Some objections will assuredly be made against the new 

 view we have offered of the mineral species, and the chief of 

 these objections must be answered in a few words. First, the 

 great number of crystalline forms belonging to many of the mi- 

 neral substances ; secondly, their very small relative interest or 

 importance; thirdly, the difficulty and complication attending 

 the description, and even the definition, of many of these crys- 

 talline forms, will be objected against their being made the types 



