302 Remarks on Tests for Cobali. 
ble to be confounded with Arsenical Fron; the latter is 
crystallized in four-sided prisms, terminated by diedral 
summits; and the ore in question is in four-sided prisms, 
also terminated by diedral summits, in many cases. In the 
arsenical iron these summits stand on the acute lateral edg- 
es, but in the ore in question, I am greatly deceived if these 
summits do not stand on the obtuse lateral edges; and some 
of the crystals are terminated by four-sided pyramids. 1 
have not been able to find any octahedra, but have found 
rhombs.’ Now can these forms, in this instance, arise except 
fromm deeply truncated octahedra?—but this I submit to 
crystallographers. . 
The presence of cobalt, under all circumstances, cANNOT 
be detected by fusion with borax; if oxide of iron, in com- 
paritively large portions, be mixed with cobalt, the colour 
communicated to borax before the blowpipe is, as you found 
it, ‘only green, or reddish, according to the degree of di- 
lution;?? and consequently this test of cobalt is not deserving 
of confidence when such quantities of iron are present; in- 
deed, I am disposed to believe that it is of no value, when 
equal parts of iron and cobalt are mixed. Stromeyer, you 
recollect, found more than 74 per 100 of iron in an arsente- 
al cobalt which he examined; and I am confident that, by 
the fusion of that ore with borax, a blue colour would not 
be communicated. {t has always appeared to me a great 
misnomer to call such minerals ‘‘ arsenical cobalt,” when 
more than two thirds of them are iron! . 
The following experiments I regard as decisive respect- 
ing the value of borax, before the blowpipe, as a sufficient 
test for cobalt when iron is present in comparatively large 
portions. 
1. Take a small bead of borax, rendered blue by a sub- 
stance known to be cobalt, and fuse it before the blowpipe 
with alittle oxide of iron: the blue colour instantly vanshes, 
and is replaced by a “ green or reddish colour, according 
to the degree of dilution.” 
2. The same effect is produced by fusing a particle of 
smalt with oxide of iron, if the colour be sufficiently diluted 
to be distinguished. _ 
3. Reduce a minute portion of cobalt to powder, mix it 
with about an equal quantity of protoxide of iron, and expose 
