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X. On the Alteration which Light produces on Red futph li- 

 ra ted Jfrferiic* known under the. Name of Realgar. Read 

 in the French Injlitute by B. G. Sage, Director of the 

 Fhjl School of Mines *. 



L 



lIGHT alters the red colours of oxides or metallic calces. 

 Minium as well as red oxide of mercury, when expofed to 

 the light, becomes black, though in clofe glafs jars. 



One of the moft celebrated cheiniits, Scheele, made known 

 that muriate of lilver loll its white colour by light even whni 

 under water. The fame phiiofopher, having decomposed the 

 light by means of a priim, found that muriate of filver, here- 

 tofore known under the name of Luna cornea, was coloured 



In 4 leconds, by - the violet ray ; 



In 25 leconds, by - the purple ray; 



In 29 feconds, by - the blue ray; 



In 37 feconds, by - the green ray; 



In 5 minutes, by - the yellow ray; 



In 12 minutes, by - the orange ray ; 



In 20 minutes, by - the red ray. 

 Native realgar, which is found in considerable maflTes among 

 the volcanic productions of Japan, is of a brilliant purple 

 red when it has been polifhed. Laroc mattes of it are found 

 aiib in ihe tin mines in the province of Kianfu, five days 

 journev from Nankin. Le Camus had a piece of it which 

 weighed twenty-eight pounds* The fame naturalift has 

 fome worked realgar, in the centre of which there is a vein 

 of calcareous fpar. The Chinefe and Indians employ it for 

 making pagodas and vafes. Having put one of thefe pagodas 

 into a glafs cafe, to which neither the fun nor external air 

 had any aceefs, I found, fome months after, that the pagoda 

 had loft its brilliancy and its red colour, and was covered 

 with an efflorefcence of an orange-yellow colour, which 

 readily detached itfelf and fell on the Hand : having collected 

 this effloreicence, and fufed it in a crucible, it prefented 

 realgar. 



I have made known that orpiment, or yellow ore of arfenic, 

 the flakes of which are of a brilliant golden-yellow colour, 

 and femi-lranfparent, panes to the (late of realgar, or Sul- 

 phurated red ore of arfenic, when fufed or fublimated. I 

 have prefented to the Inftitute a piece of orpiment, a part of 

 which I have made to pal's to the Hate of realgar by applying 

 beneath it the flame of a taper by means of a blowpipe : for 

 this purpofe, the flame mult not be too Itrong. 



* From the fame. 



Pott 



