[ 22 ] 
IRON. 
IRON, being of all metals the moft imperfedt, 
is fubjedt by various means, to be calcined or re- 
duced to a ruddy crocus, fimilar to the ruft that' 
arifes from it’s being corroded by the acid in the 
air. In this date, it requires a confiderable degree 
of heat to diffolve and incorporate it with glafs ; 
till that heat is applied, it retains it’s ruddy colour ; 
by increafing the heat, it palTes through the inter- 
mediate colours, till it arrives at it’s permanent one, 
which is blue: this being effedled in the lame de- 
gree of heat in which we liave examined the other 
metals, that is, the great eji that the glafs will bear 
without lojing all colour whatever. 
The green, with which the glafs ufed for bottles 
and chemical veffels is tinged, is occafioned by the 
iron contained in the vegetable alhes and fand, of 
which that glafs is compofed. When the pots, in 
which the matter has been kept in fufion, are 
Kunkel makes the following remark on it: “ This compofition 
“ is very difficult to make \ it is necelTary to feize the moment 
“ at which the matter is well tinged red, to take it immediately 
“ from the fire ; for half a quarter of an hour too much is luffi-- 
c;ent to change tf s colour’* In the next chapter, fpeaking of a 
red enamel, into the compofition of which copper enters among 
other ingredients, Kunkel makes this remaik . T his ccmpo- 
fition is very fine, and lefs troublefome than the prcceuiiig one. 
“ but, after having added the copper, the matter muft not be left 
“ on the fire ; if this is not attended to, it becomes green, and the 
“ red colour that it had taken at firlt does not lojL” 
Gcllert, Chem. Mctallur. problem 97. Copper gives a blood- 
red to glafs; but if it is left too long on the fire, it becomes green,^ 
nearly 
