. 25. PRELIMINARY OBSERVATIONS. 25 



different kinds of lustre, the regukr forms, the circum. 

 stances and relations, under which the particles of the in- 

 dividuals can be separated from each other, &c. ; because 

 these are the properties of minerals exhibited in their na- 

 tural state, and may be considered without producing any 

 change or alteration on the mineral. 



Properties which can only be observed during, or after 

 a change, cannot be employed agreeably to the principles 

 of Natural History, and must therefore be excluded from 

 Mineralogy ; because, in observing them, we transfer the 

 object itself from its natural state, into another, in which it 

 ceases altogether to be an object of Natural History, Pro- 

 perties of this kind are, the fusibility of minerals examined 

 before the blow-pipe, or by the assistance of some other 

 apparatus, and the concomitant phenomena ; their solu- 

 bility in acids ; phosphorescence produced by heat, if, after 

 the first experiment, it cannot be observed any longer ; 

 chemical analysis instituted to ascertain the quality or re- 

 lative quantity of the component parts, and the results 

 of that process : every thing, in short, must be excluded, 

 which alters the natural state of a mineral. There are 

 properties to be met with in minerals in their natural state, 

 which, although not altered by examination, yet are of no 

 utility in Natural History ; such as the size of crystals ; 

 the irregular enlargement, and figure of some of the faces 

 depending upon it ; the accidental forms minerals assume 

 by being broken, rubbed down, water-worn, decomposed, &c. 

 Such properties are accidental, because the identity (. 14.) 

 of the individual is not destroyed by their occurrence. 



The natural-historical properties include the greater part 

 of the characters commonly called external, and some of 

 those called physical. 



As to the distribution of those properties among the dif- 

 ferent heads mentioned above, the first will include those 

 which can be observed only in an individual itself, or in a 

 fragment of an individual. These are the geometrical pro- 

 perties, or such as refer to Space ; the relations of Struc- 

 ture, those of Surface, and the phenomena of Refraction, 

 in so far as they depend upon the regular form of minerals. 



