Treatise on Poisons by Sir Robert Christison

Blue vitriol, blue copperas, blue stone, vitriol of copper, as it is variously called in common speech, is the sulphate of copper. In the solid form it constitutes large crystals of a deep blue colour, and an acrid, astringent, metallic taste, efflorescent in dry air, and very soluble in water. Under the action of heat it first loses its water of crystallization without undergoing the watery effusion; then its sulphuric acid is driven off partly unchanged, partly decomposed; and at last the brown peroxide is left behind in a state of considerable purity. If carbonaceous matter be previously mixed with the sulphate, the oxide is decomposed at a low red heat, so that the process of reduction may be performed in a glass tube. For the reasons formerly stated, this process does not constitute a convenient or characteristic test for sulphate of copper. The best mode of ascertaining its nature is to dissolve it, and then to apply the tests for the solution. There are many excellent tests for copper in solution. But the four following are the most delicate and characteristic,—ammonia, sulphuretted hydrogen, ferro-cyanate of potass, and metallic iron.