viii PREFACE, 



je&ural art. This will plainly appear by ex- 

 amining each of the external characters in 

 particular ; namely, colour, tranfparency, or 

 opacity, coherence, texture, fhape, andfpe- 

 eific gravity. 



And firft as to colour^ Mr. Werner ownes 

 that white quartz, white lead ore, and white 

 calcareous iron ore, have exaffly the fame 

 fnoiv 'white colour. Some fpecies of the ores 

 of iron, manganefe, cobalt, and copper, the 

 fame iron grey colour, wolfram and blende 

 the fame broivnijh black^ &c. in fhort it is 

 well known, that the flighted change in the 

 texture of bodies, frequently produces a con- 

 fiderable alteration in colour ; thus a lump 

 of cinnabar, whofe colour is dark red, becomes 

 of a beautiful florid red, by fimply reducing 

 it to a powder; befides.it is difficult, if not 

 impoflible, to render the various fhades of 

 colour intelligible by any defcription. 



Tranfparency and Opacity are qualities com- 

 mon to a vaft variety of fubftances, different 

 in all other refpedls. They are fufceptible of 

 numerous undifcribable degrees, and difcover 

 at moft the mode of union, not the fubftan- 

 ces united. 



I V, 



Coherence andhardnefs are properties equally 

 ambiguous : with refpeft to earths and pow*- 



deredi 



