Terma-nent Colours of Opake Bodies. 223 



Its fpecific denfity, yet as it differs greatly in that 

 refpeft, from the denfer metallic powders, the 

 moleculas of oil, which intercede their particles, 

 aft upon the incident light fo ftrongly that the 

 reflexions, efFeded by them, cannot be diftin- 

 wuifhed from thofe which are caufed by rarer 

 media. 



When ochres, and other paints, which owe 

 their colours to the rarer metals, are combined 

 with oil, they aflume a darker hue. Becaufe the 

 excefs of the denfity of oil, over that of air, forms 

 a fenfible difference, when comparatively confi- 

 dered, with refpecft to the fpecific gravity of the 

 rarer metals. From this caufe, perceptibly lefs 

 light is reflefted from the moleculse of oil, than 

 from thofe of air> and confequently the mafs 

 appears darker. 



When air is expelled from, and oil is united 

 with, coloured powders, whole fpecific denfity is 

 much lefb than that of metallic calces, the 

 brightnefs of their colours is diminifhed in 

 proportion as the powders approximate, in fpe- 

 cific denfity, to the media which occupy theif 

 interftices. And when the fpecific denfity of 

 thefe is nearly equal, there is no fenfible re- 

 fledtion in the confines of their refpeflivefurfaces, 

 and therefore the mafs appears black. 



Thus, when indigo, and other tranfparent 

 vegetable paints^ are united with oil, the air is 

 thereby expelled from their interftices ; and the 



oil. 



