C'hap. 29.] Green Vitriol 167 



lutibn of iron in the vitriolic acid is of a green colour, 

 and by evaporation produces the green cryftals, well 

 known under the names of fal martis> green vitriol', 

 and copperas. The green vitriol ufed in commerce is 

 obtained in the following manner : Pyrites, which are 

 natural combinations of iron and fulphur, are expofed 

 to the a<5bion of the air and rain in mallow pits lined 

 with clay. Afcer having been in this fituation a week 

 or a fortnight they grow hot and crumble down, and 

 when carefully examined are found to ; contain- fmall 

 cryftals. Thefe are difiblved by the rain, arid con- 

 veyed by pipes into a refervoir in a houfe, whence the 

 liquor is pumped into a* boiler made of lead. This 

 liquor is found to 1 have an excefs of acid, which is re- 

 medied by calling pieces of iron into it when heated 

 fo as to firnmer. By the addition of the iron a quan- 

 tity of the earth of alum is alfo depofited. As the 

 liquor cools, the greater part of the fal martis is depo- 

 fhed. By the expofure of the pyrites to the action of 

 air and water, the fulphur attracts the oxygenous prin- 

 ciple, and is thus converted into vitriolic acid. Sal 

 martis is liable to the watery fufion; when expofed to 

 a ftrong heat the acid begins to exhale, and as it ex- 

 hales, the natural colours of the calx of iron appear. 

 It is firft yellow, then orange, then red j if it is cal- 

 cined to a greater degree, fcarcely any of the acid is 

 lefc, and the calx remains of a deep purple colour, and 

 is known under the name of cokothar of vitriol. The 

 nitrous acid acts with fo much violence on iron as 

 to convert it into a brown calx. With the muriatic 

 acid iron affords cryftals of a livelier green colour than 

 copperas, which will not, like copperas, part with the 

 acid by the application of heat. 



All folutions of iron, if expofed to the air, depofit 



part of the metal in the form of a calx. Alkaline falts 



M 4 precipitate 



