COPPER. 
§§ 832 - 834 .] 
access of air, ammonia also acts as a slow solvent. Metallic copper in 
a fine state of division can be fused at a white heat to a bright bluish- 
green globule, which, on cooling, is covered with black oxide. 
§ 832. Cupric Oxide, CuO=79-5—specific gravity, 6-5 ; composition 
in 100 parts, Cu 79-90, 0 20-10—is a brownish-black powder, which 
remains in the absence of reducing gases unaltered at a red heat. It 
is nearly insoluble in water, but soluble in C1H, NO s H, etc. ; it is hygro¬ 
scopic, and, as everyone who has made a combustion knows, is readily 
reduced by ignition with charcoal in the presence of reducing gases. 
§ 833. Cupric Sulphide, CuS = 95-66, produced in the wet way, is 
a brownish powder so insoluble in water that, according to Fresenius, 
950,000 parts of water are required to dissolve one part. It is not 
quite insoluble in C1H, and dissolves readily in nitric acid with separa¬ 
tion of sulphur. By ignition in a stream of H it may be converted 
into the subsulphide of copper. It must always be washed by SH 2 
water. It is slightly soluble in the alkaline polysulphides, especially 
in the presence of sulphides of arsenic, antimony, and tin. 
§ 834. Solubility of Copper in Water and Various Fluids. —The 
solubility of copper in water and saline solutions has been very care¬ 
fully studied by Carnelley. 1 Distilled water exerts some solvent action, 
the amount varying, as might be expected, according to the time of 
exposure, the amount of surface exposed, the quantity of water acting 
upon the copper, etc. It would appear that, under favourable circum¬ 
stances, 100 c.c. of distilled water may dissolve -3 mgrm. of copper 
(-2 grain per gallon). 
With regard to salts, those of ammonium exert a solvent action on 
copper more decided than that of any others known. With the others, 
however, the nature of the base exerts little influence, the action of the 
salt depending chiefly on the nature of its acid radical. Thus, beginning 
with the least effective, the following is the order of dissolving strength 
—nitrates, sulphates, carbonates, and chlorides. It will then at once 
be evident that a water contaminated by sewage, and therefore con¬ 
taining plenty of ammonia and chlorides, might exert a very considerable 
solvent action on copper. 
Almost all the oils and fats, as well as syrups, dissolve small 
quantities of copper ; hence its frequent presence in articles of food 
cooked or prepared in copper vessels. In the very elaborate and careful 
experiments of Mr W. Thompson, 2 the only oils which took up no 
copper, when digested on copper foil, were English neats’-foot oil, 
tallow oil, one sample of olive oil, palm-nut oil, common tallow oil, and 
white oil, which was protected from the air by a thick coating of oxidised 
oil on its surface. 
1 Journ. Chem. 80 c., 1876, ii. 4. 
2 “ Action of Fatty Oils on Metallic Copper,” Chem. News, xxxiv. 176, 200, 313. 
