258 Mr. Delaval on the Caiije of the 



refled all the forts of rays, and thereby become 

 white. 



Of this kind are the air, or other rare fluids, 

 which occupy the interftices of liquors; of tranf- 

 parent folid bodies, flawed, or reduced to powder, 

 or othervvife minutely divided ; of porous folid 

 fubfliances; of calces of metals; and in general 

 of all denfer media, into whofe interftices fuch 

 rarer particles are admitted. 



Thefe obfervations are grounded upon Sir Ifaac 

 Newton's doflrine, relative to the tranfparency, 

 and opacity of Colourlefs Subftances. And from 

 thence it may be deduced, that white opake bo- 

 dies are conftituted by the union or contiguity of 

 two, or more, Tranfparent Colourlefs Media, dif- 

 fering confiderably from each other, in their re- 

 flective powers. 



Such white fubftances are inftanced in froth, 

 emulfions, or other imperfefl combinations of pel- 

 lucid liquors, milk, fnow, falts calcined or pul- 

 verized, glafs, or cryfl:al, reduced to powder, 

 white earths, paper, linen, and even thofe metals, 

 which are called white by the mineralogifts and 

 chymifts. For thofe metals do not appear white, 

 unlefs their furfaces be rough. Becaufe, the fur- 

 faces of polifhed metals do not affbrd interftices, 

 into which air, or other rare fluids capable of re- 

 fledling light, can be admitted. 



But, when interftices are opened on the furfaces 

 of metals, by roughening them, the air occupies 

 ihofc interftices, and its molecular, according to 



the 



