Chap. 9.] Colours of opake Bodies. 275 



By polifhing the furface alfo, the air is removed from 

 them, and the dark hue is reftored." 



From thefe experiments he concludes, " that vege- 

 table, animal, and mineral coloured matter is tranfpa- 

 rent j that it does not reflect colours, but only exhibits 

 them by tranfmiffion ; that opake coloured bodies 

 confift of tranfparent matter covering opake white par- 

 ticles, and tranfmitting the light which is reflected 

 from them." 



With refpeft to the femi-pellucid fubftances, fuch 

 as the folution of lignum nephriticum, &c. which ap- 

 pear of one colour by incident and another by tranf- 

 mitted light, he fays, " they confift of pellucid media* 

 through which white or colourlefs opake particles are 

 diffufed." Thefe white particles, he adds, are difpofed 

 at fuch diftances from each other, that fome of the in- 

 cident rays of light are capable of patting through the 

 intervals which intercede them, and thus are tranfrr.it- 

 ted through the femi-pellucid mafs. Some forts of 

 rays penetrate through the mafles, whilft other fortsj 

 which differ from them in refrangibility, are refracted 

 by the white or colourlefs particles. Thus, when pel- 

 lucid colourlefs glafs is melted with arfenic, the arfenic 

 is thereby divided into minute opake particles, which 

 are equally diffufed through the glafs. If only a fmall 

 quantity of arfenic is ufed in this compound, the white 

 particles are thinly difieminated in it. When glafs of 

 this compofition is held between the window and the 

 eye, it exhibits a yellow or orange tinge when viewed 

 by incident light it is blue. The yellow or orange 

 arifrs from the lefs refrangible rays, from the mixture 

 of which that colour refults. The mere refrangible 

 are reflected back by the white particles. 



