[ 33 J 
rron diffolved by the vitriolic acid is' green ; bat 
if further attenuated by a chemical procefs, it pro- 
duces that beautiful colour called 'Prujjiaii blue, 
Phil. Tranf. N®. 38. Henckel, Differt. 6. 
A fimilar blue may be obtained from the iro 7 i 
contained in the alhes of all plants. Henckel, 
Flor, fat. chap. 8. parag. 55. 
Having expofed a pound of wood aflies in a luted 
crucible, to a pretty Ifrong fire, for thirty hours, 
the greatefl part of them became tinged by the 
iron contained in them. 
A blue may be alfo extradled from a martial 
*vitriolj by fpirit of wine. Henckel, de appropria- 
tione, chap. 2. parag. 257. 
An inftance of a mineral fubflance changing it’s 
colour from green to blue on it’s fpecijic gravity 
being dhninijhedf appears in a ftone defcribed by 
Dr. Grew in the Mufeum of the Royal Society : 
this gem is a kind of emerald, which, when expanded. 
by heat, becomes blue, and remains of that colour 
till cold, in which ftate it returns to it’s ufual 
colour, which is green. 
Tin is not capable of being vitrified, or impart-' 
ing any colour to glafs ; nor are any preparations 
of it of any primary colour. 
F 
VoL. LV. • 
MERCURY. 
