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ON ARTIFICIAL BLUE COPPER. 
ART. LXIL— ON ARTIFICIAL BLUE COPPER, (COPPER- 
INDIGO.) 
By A. and C. Walter. 
The monosulphuret of copper, hitherto artificially ob- 
tained either by precipitating persalts of copper with sul- 
phuretted hydrogen or by treating the sesquisulphuret of 
copper with cold concentrated nitric acid, is generally not 
blue like copper-indigo, but dark-green or brownish-black; 
if, however, the precipitate obtained with sulphuretted hy- 
drogen is dried at a moderate temperature, it frequently 
yields a greyish-black powder, which under a steel polisher, 
gives a faint steel-blue streak, especially when it is fixed 
by means of size upon paper, and after drying burnished 
with the polishing steel. According to the observations of 
Covelli respecting the formation of blue copper at Vesuvius, 
it appears as if it were produced by the long-continued 
action of sulphur upon persalts of copper; and the authors 
therefore attempted to prepare it in a similar manner. After 
numerous experiments, they found the following process to 
be the most advantageous Pure sulphate of copper is dis- 
solved in hot water, dilute solution of caustic potash or soda 
added to it until nothing further is precipitated; the precipi- 
tate is then washed in hot water, and dried at a high tem- 
perature, until the entire mass has become converted into 
black oxide. 1 part of this oxide is now heated in an open 
dish with 1 part sulphur and \ part chloride of ammonium 
until the sulphur takes fire; while the sulphur burns, the 
mixture is well stirred, covered from time to time with a 
loose lid, removed from the fire for a time, and afresh quan- 
tity of 1 part sulphur and \ part chloride of ammonium 
added, the dish covered and again exposed to a gentle heat. 
When the dish has been left covered for some time, it is re- 
