MINERALOGY. 5$ 



ilones, among which they elfe might have been 

 ranked equally as here, as they are already in 

 other fyftems : to which I venture to add, that, 

 as a naturalift, I cannot conquer that general 

 weaknefs of valuing them fo highly \ for befides 

 their furprifing hardnefs, and line colours, that 

 pleafe the eve, it is not without foundation, that 

 they might be thought applicable to every ufe 

 for which the filiceous kind is employed, if they 

 were to be had in large quantities r and by this 

 alone it is they deferve to have the preference of 

 the other flones of this order. In regard to the 

 colours, it is to be obferved, that thofe of 

 the ruby and emerald are faid to remain in the 

 fire, but that the colour of the topaz flies off: 

 whence it is ufual to burn that gem on purpofe 

 that it may be made ufe of inftead of the diamond, 

 as it is harder than the quartz cryftal. The 

 colours of gems are commonly fuppofed to de- 

 pend on metallic vapours : but may they not 

 more juftly be imagined to arife from a phlo- 

 gifton, united with a little metallic or ibme 

 other earth r becaufe we find that metallic earths, 

 which are perfectly well calcined, give no colour 

 to any glafs, and that the manganefe, on the 

 other hand, gives more colour than can be 

 afcribed to the fmall quantity of metal which 

 is to be extracted from it. (Section cxiii.) 



The phlogifton may perhaps have more diffi- 

 culty to efcape through the pores of the harder! 

 of the precious flones, if it is true, that the pro- 

 perty of lofing the colour is in proportion to 

 their hardnefs, as fome authors feem to in- 

 dicate, by alluring us, that none but the co- 

 loured diamonds and the rubies keep their co- 

 lours in the fire ; but in this circumftance I like- 

 wife want experience, but hope to fee it illuftrated 



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