Permanent Colours of Opake Bodies, 263 



fmoothnefs, they refemble the polifhed furfaces 

 of metals. 



Metals feem to confill entirely of Tranfparent 

 Matter, and to owe their apparent opacity, and 

 ludre, folely to the copious refledlion of light from 

 their furfaces. The analogy between the metals 

 and Tranfparent Media, as far as refpedls their 

 optical properties, will appear from the following 

 confiderations. 



(i) All metals, difiblved in their proper men- 

 flrua, are tranfparent. (2) By the union of two 

 or more tranfparent media, fubftances are confti- 

 tuted, which are fimilar to metals, in their opacity 

 and luftre, as has been inftanced in plumbago, 

 and marcafites. (3) The Tranfparent Subftances 

 of metals, as well as of thofe minerals, by their 

 union with Phlogifton, acquire their (Irong re- 

 fledive powers, from which their luflre and opa- 

 city arife. (4) The furfaces of pellucid media, 

 fuch as glafs, or water, afTume a metallic appear- 

 ance, when by their fmoothnefs, their difference 

 of denfiry with refped to the contiguous media, 

 or any other caufe, they are difpofed copioufly to 

 reflect the light. 



From all thefe confiderations, it is evident, that 

 opake fubftances are conftituted by the union, or 

 contiguity, of Tranfparent Colourlefs Media, dif- 

 fering from each other in their refledive powers; 

 and, that, when the common furface, which inter- 

 cedes fuch media, is plane, equal, and fmooth, it 



S 4 refleds 



