[ 24 ] 
'hours to a red heat. When this mixture was cold, 
1 feparated from it, by a magnet, fmall grains of 
trorii weighing about one twentieth part of the 
fand. 
Exp. 3 . I melted fand thus deprived of it's iroti, 
v/ith half its weight of borax and the fame quan- 
tity of nitre, and found that it produced a per- 
feftly colourlefs and tranfparent glafs. 
Exp. 4. To two parts of the white fand ufed in 
making cryftal glafs, and one of 'borax and nitres 
I added a twentieth .part in weight, of the grains 
of irony which I had extrafled from the fand by 
Exp. 2. and having vitrified this compofition, 1 
found that it was become exactly fiinilar in colour y 
to that commonly ufed in making green glafs. 
Exp. 5. I expofed feveral pieces oi green bottle 
glafs made at different glafs-houfes, under a muffle, 
to a ftrong 'fire, for the fpace of half an hour, and 
found that they were all become blue. 
If the crocus of iron is added in too great a pro- 
portion, it continues to adhere together, and re- 
mains unmixed, or at lead; imperfectly mixed, with 
the glafs, retaining for that reafon the colour 
natural to it when undifiblved; or if it be in a 
fmaller quantity, though yet in too great a pro- 
portion to be diffolved, it will make fome inter- 
mediate colour between the ruddy and the blucy 
which lafi; it always imparts, when iii a fufficient 
degree of fire and a proper proportion. The ne- 
ceffity of a due proportion of metal to the glafs has 
been already infianced in gold, which if in too 
large a proportion to be difiblvcd by the glafs, in- 
flead 
