Permanent Colours of Opake Bodies, 219 



tained in the pores of Tranfparent Coloured 

 Subftances. 



I^ is evident that calces, feparated from me- 

 tallic folutions, derive their power of exhibiting 

 Coloured Light from the fame caufes, which 

 prevail in thofe inftances which I have here 

 adduced, and explained. For, in proportion 

 as the interftices of the calces are evacuated, 

 by the deprivation of their liquid folvents, the 

 air pervades thofe vacuities. 



Vitriols, and other metallic falts, are formed 

 from metallic folutions, by the evaporation of 

 their aqueous part, excepting only fuch a por- 

 tion of it as is eflential to their cryftallization. 



Thefe metallic falts are either coloured, or 

 colourlefs, according to the folutions from which 

 they are formed. Whilft their conftituent prin- 

 ciples remain united in due proportion,^ and 

 order, they are pellucid. But when their watery 

 part is difengaged, they lofe their tranfparency, 

 and texture, and are reduced to an opake friable 

 mafs, or to powder. 



In this ftate, air pervades the interftices which 

 the water had evacuated, and reflefts a white 

 light, in confequence of the difference of its 

 denfuy from that of the matter which is con- 

 tiguous to it; and as even the coloured vitriols, 

 and other coloured metallic falts, contain a large 

 proportion of colourlefs faline matter; when the 

 water of their cryftallization is expelled, their 



metallic 



