On Crystallography, 343 



the crystals, sometimes transversal, or perpendicular to the 

 same axis, which also tends to rctcr it to the geometrical 

 charactcTS. Perhaps it may be said that it would have 

 been more agreeable to change the word geometrical into 

 another, which would liave mdicated in a looser manner 

 the modifications depending on the configuration. Bu; 

 this word is so well adapted to those of" physical character 

 and che7nical character, that we have preferred preserving 

 it, by explaining tlie signification which it ought to havt 

 in the language ot mineralogy. 



The chemical characters are those which are proved by 

 the decomposition of a mineral, or a sensible alteration in 

 its nature, or a rupture of aggregation between its molecules. 

 Such are the characters which are drawn from the action 

 of the acids, from fusion wiih or without addition, by the 

 intermedium of the blowpipe, &c. 



It is from the assemblage of these three orders of cha- 

 racters that the character will be formed which we call 

 specific^ or that which will serve to distinguish all the bodies 

 comprehended within one and the same species. We 

 ought not to be afraid, from the reaspns which we have 

 already given, of multiplying the particular indications of 

 which it is the assemblage, in order to procure the facility 

 of making them serve for mutually verifying each other, 

 or even of substituting the one for the other. 



But is it not also an inconvenience, that the table of the 

 characters of a mineral is so overloaded, that we are obliged 

 to run over the whole of it without fixing on any thing 

 which can give a precise knowledge of this mineral, and 

 assist the mind in representing it as it were in miniature?' 

 It is with a view to obviate this inconvenience that I have 

 adopted a character which 1 call essential, and which is 

 composed of the smallest possible number of particular 

 characters taken from among those of the species which are 

 proper for distinguishing the latter from all others. Thus 

 the essential of the teltsia consists in its having a specific 

 gravity of about 4, and presenting joints only in a direction 

 perpendicular to the axis of the crystals : — that t)f the cha- 

 basy consists in its dividing into a rhomboid a little obtuse, 

 and it mtlls easily in the -blowpipe : — that of borated mag- 

 nesia consists in the crystals of this mineral being electrical 

 by heat in eight points opposed to each other in pairs; — 

 that of sulphurated molybdenum is to leave metallic traces 

 on paper, and to conmiunicate electricity to resin by fric- 

 tion. The characters which compose what I call essential^ 

 will not be observable in all cases : but it will be always 



Y 4 correct 



