On Cryslallography* 35 i 



which enters into this character ought to be so much the 

 niore taken into the account, the purer the substance is, or 

 the more it approaches the hmit which really constitutes 

 its species : and that in the case of this limit the colour 

 would disappear*. 



As to the diversities oF which the character is suscepti- 

 ble that is derived from colours, it would be superfluous to 

 enumerate them, because we find on this head in daily ob- 

 servation, and in the commonly received language, every 

 thing that can be required by science and its nomenclature. 



Thus, in order to designate any. given shade of colour, 

 sometimes we add a simple adjective ; as when we say pure 

 green, bright red, dark blue : sometimes we refer the colour 

 to a term of comparison taken from among familiar ob- 

 jects ; as when we say sky blue, saffron yellow, leek green, 

 &c. : on other occasions we give the two colours of which 

 the object in question seems to partake, and say, for ex- 

 ample, greenish yellow, or yellowish green, by calling in 

 the predominant colour first. 



9. Cat's eye colours. This expression alludes to the eyes 

 of a cat which shine in the dark. We say of a sub- 

 stance that it is cat's eyed, when, in proportion as we vary 

 the position of its surface, the reflections of light which it 

 iiives off are in some measure moveable, or appear and 

 disappear alternately. 



JO. Metallic lustre. We may distinguish the true from 

 that which is apparent only, in so far as the mark of a file 

 or any sharp instrument with which we have scratched a 

 metal does not cease to shine, whereas it is dirty, and as it 

 were powderv, when the body is not of a metallic nature. 



11. Limpid bodies. VVe have given the name oi white 

 to diaphanous and colourless minerals properly so call- 

 ed, such as the (j^w2^x\z Q.d\Wi\ Madagascar crystal. We 

 shall cajl these limpid minerals, and reserve the deno- 



♦ We must confess that there would have been some advantage in quot- 

 ing at the head of the description of a Species, the character derived from 

 the modiiication in qaerion, it the j^reater number of the varieties presented 

 cne and the same colour, or nearly so. in such a way that the difffrences 

 which would have taken place in other varieties might have been regarded 

 as exceptions; because, this character being that which first strikes the eye, 

 its indication would tb.eieby be very proper for becoming as if the first 

 stroke of the pencil in giving the portrait of a mineral. But if we are 

 obh'ged to quote :\t onct- eight or ten diflereat colours, which are shajed by 

 various individuals of the species, wiil it not appear that the dcscriptioa 

 commences by falling into absurdity, and by failing in its principal object, 

 which is to admit of a facilitv of ascertaining at one glance the »ubstance 

 indicated ? ' ^ 



Z 2 mination 



