[ 25 ] 
ftead of imparting a red colour to it, runs together 
in it’^s metallic form. 
Henckel has given us a method of making a 
beautiful blue glafs by this means. It con fills 
merely in mixing iron with the matter of which 
the pur eji glafs is compofed, and expofing it to a 
violent Jire. Gellert obferved alfo, that iron imparts 
to glafs this colour |_y]. Mr. Lehman obtained 
the fame colour from emery, which is a kind of 
iron ore^ or ferruginous ftone, by mixing it with a 
vitrifiable earth ; which colour he attributes to the 
iro7i contained in it [ 2 ;]. Neri mentions a fky colour 
[y'j Henckel, Defect. 6. On a blue obtained from iron. 
1 coloured glafs with iron, and gave it a very fine blue. I had 
calcined, in a pot, hleings of Styrian fieel, which I kept about a 
quarter of an hour in the fire, without fiii ring them, till they had 
taken a purple colour, bordering on violet ; I mixed about half 
a grain, which I had ground well in a glafs mortar, with fifteen 
grains of very white flint and a very fine alkali ; I put the whole 
into a crucible, covered it with care, and expofed it to the inoji 
violent fire. The furnace being cooled, I found a glafis of the 
colour of a fiapphire j it was impoflible to fee a finer blue, cither for 
colour or tranfparence. 
Gellert. Chem. Metallurg. vol. ii. prob. 97. Cobalt calcined 
gives to glafs a very fine blue ; but if too much cobalt, for exam- 
ple an eighth part, .is ufed, the glafs’w'ill become black. The 
calx of iron produces the fame efifeSi ; and fometimes it happens 
that one fide of a thin piece of glafs is of the colour of r/y?, while 
the other fide is blue. 
[z] Lehman, Treatife on the formation of rhetals, p. 37. 
As to the earth of cobalt, or bifmuth, which is the bafis of the 
blue colour, Henckel confidered it as a martial earth in his Opufe. 
miner, p. 573. Some experiments which I have made render 
that opinion very probablei I have obtained a very fine blue 
colour from 2l ferruginous Spanijlh emery : having once pulverifed 
half a pound thereof and mixed it with an equal part of black 
flux, I melted this mixture iaa well-covered crucible in a pretty 
VoL. LV. E' imparted 
