2 1 6 Mr. Belaval on the Cau/e of the 



proportion to the white particles of the gold, as 

 that part of the yellow-making rays, which were 

 intercepted, bears to all the other rays comprized 

 in the white light of the fun. 



Calces are metals, diveded of their phlogifbic 

 principle, by menftrua, or by fire. The calcined 

 particles are fufpended in their folvents, and, to-, 

 gether with them, form tranfparent liquors. The 

 metals, thus difTolved, refleft no colours, but 

 either tranfmit coloured light, or are pellucid and 

 colourlefs. 



It is proper here to explain the caufe, why the 

 metallic particles, which yield no colour, by in- 

 cident light, when fufpended in their folvents, are 

 difpofed to exhibit colours, when feparated from 

 their menftrua, and reduced to a cakiform ftate. 



Sir Ifaac Newton has fhewn, that when fpaces, 

 or interftices, of bodies are replenifhed with media 

 of a different denfity, the Bodies are Opake-, that 

 thofe fuperfices of tranfparent bodies refled the 

 greateft quantity of light, which intercede media 

 that differ mofl: in their refradive denfities; that 

 the rePiedions of very thin 'Tranfparent Subflances 

 are confiderably flronger, than thofe made by the 

 fame fubHances of a greater thicknefs. 



On thcfe grounds, the minute portions of air, 

 or of the rarer medium, which occupies fpaces void 

 of other matter, refieft a vivid IVbite Light, when- 

 ever their furfaces are contiguous to media, whofe 

 denfities differ confiderably from their own. So 



that. 



