(3) Copperas. Where beds of common iron pyrites occur, the iron sulphide is converted into sulphate by exposure to the oxidising influence of the air. The result is an acid sulphate of iron, which is leached out and neutralised by addition of more iron in the form of scrap. The neutral sulphate is crystallised out of the liquor, and calcined in a muffle furnace, the shade of the ultimate product being governed by the degree or duration of the roasting. The sulphurous acid liberated in the roasting is sometimes utilised for making sulphuric acid, but is more often wasted, because, to be commercially successful, the sulphuric acid manufacture must be conducted on a large scale, demanding 100_l._ of capital for every 1_l._ necessary for the copper and red oxide fabrication. PERSIAN RED.--A name which is used somewhat indiscriminately both for Derby red (p. 145) and for oxide red (p. 150). REALGAR.--The native mineral realgar is a yellow-red bisulphide of arsenic, often called also ruby of arsenic, or arsenic orange. It occurs native in very limited quantities in some of the older rocks, and then only requires to be ground and levigated. But for painters’ purposes it is prepared artificially by heating a mixture of sulphur and arsenic in such a way that they are melted in company and react on each other to form the arsenic sulphide. The heating takes place in crucibles, and the proportions are two parts by weight of arsenious acid (white arsenic) to one of flowers of sulphur. When the reaction has ceased, the contents of the crucible are allowed to cool, and then reduced to very fine powder. The pigment is exceedingly poisonous and not remarkably durable, besides which, it cannot be mixed with any other pigment which is affected by sulphur. RED LEAD.--A common name for minium, see p. 148. ROUGE.--One of the names for a particular shade of the oxide reds, see p. 150. VENETIAN RED.--A fancy name for a special shade of oxide red, see p. 150. VERMILION.--This old pigment is gradually going out of use; the newer reds, which are more brilliant in colour and cheaper, are gradually displacing it, although it is doubtful whether it will ever go completely out of use. It is the mercuric sulphide HgS. When pure, it is not attacked by acids or alkalies; only aqua regia, a mixture of hydrochloric and nitric acids, is capable of dissolving it, when it forms a clear solution. Heated in the flame of a Bunsen burner, it is completely volatile, a property possessed by no other pigment in common use, therefore any adulteration can be readily detected by simply heating a little vermilion in a crucible; if a known weight is taken and the residue is weighed, the amount of adulteration can be ascertained. Vermilion is chiefly adulterated with oxide of iron and orange lead. From the character of the residue left on heating in a crucible, the kind of adulteration can be readily ascertained.