ON THE IMPURITIES OF COMMERCIAL ZINC. 69 
tremity of the fine tube placed under lime-water in a small test-glass. The bulb was 
heated till the chromate of lead was fused, and if any carbon had been present in the 
residue, the evolved gas would have caused a cloudiness or precipitation of carbonate 
of lime in the lime-water. In this manner we first tested our chromate of lead by 
itself, and found no cloud or precipitate in the lime-water. We next inserted the 
smallest possible particle of carbon into the bulb with a little chromate of lead, and 
obtained a large distinct cloud of carbonate of lime in the lime-water in the test-glass, 
and in the capillary tube which delivered the gas. Having thus proved the purity 
of the reagent employed, and the extreme delicacy of the test, we tested in succession 
the residues from Silesian, Vieille Montagne, and Berlin zincs, and from the zinc of 
Rousseau Fréres. As precisely the same result was obtained with each of these zincs, 
it may be stated once for all. 
On heating the mixture of residue and chromate of lead till the chromate fused, 
there appeared in each case a very slight deposit on the upper surface of the lime-water 
column in the fine tube. This deposit could not have been smaller and yet been visi- 
ble; it was incomparably less than that produced by the atom of carbon which was 
purposely introduced into a similar tube, and was undoubtedly caused by the slight 
dust which collected on the residues during the processes of washing and drying, and 
which no possible precautions could entirely avoid. It is obvious from these experi- 
ments, that the often repeated statement, that the insoluble residue from zinc treated 
with dilute acids is carbon, rests on no adequate foundations, and that carbon is not 
an invariable constituent of crude zinc, as it is of iron. ' But on the other hand it is 
impossible to assert that carbon does not sometimes occur in commercial zinc as an 
accidental and wholly abnormal impurity. Thus in the specimen of New Jersey zinc 
which we examined, there were certain small cavities lined with black, as if a bubble 
of some carbonaceous gas had been decomposed within them, and the residue from this 
zinc, when tested as above described for carbon, produced a distinct cloudiness in the 
lime-water, which was sufficient evidence of the,presence of a trace of carbon in this 
spelter; but the amount of this impurity was infinitesimal, and not ab: all to be "- 
pared in quantity with the lead and other metallic pum of which the residue 
mainly consisted. "The presence of a little copper in this zinc may perhaps be con- 
nected with the occurrence of this trace of carbon. The — im three of the 
English spelters also gave distinct reactions for carbonic ee in the lime-water; but, 
judging from the exceedingly small cloud of carbonate of lime producer the amount 
of carbon in these zincs is even less considerable than that detected in the New Jersey 
zinc. The other English zinc (that from the works of Messrs. Vivian) yielded but the 
