=Sulphuretted Hydrogen=, H₂S = 34.—This is a gas of acid properties smelling like rotten eggs; it precipitates the sulphur compounds of many metals when led into the acid solution of the corresponding salt. This substance is seldom required in colour works, so that it is convenient to have an apparatus which permits of the preparation of any required quantity. Fig. 3 represents an apparatus devised by the author, which is well adapted for the preparation of sulphuretted hydrogen. It consists of a small, wooden tub, on whose upper edge lies a thick paper ring, so that the lid may be pressed down air-tight by the screws B. Through the lid pass a tap-funnel, T, a movable screw, S, and a tube, R, to carry away the gas. On the screw S hangs a basket, K, by a handle; this is filled with pieces of iron sulphide as large as nuts. The tub is filled to about one-third of its height with a mixture of 9 parts of water and 1 part of sulphuric acid. [Illustration: FIG. 3.] When sulphuretted hydrogen is required the basket is lowered by the screw, S, until it dips in the liquid; according as the basket dips more or less into the liquid a fast or slow current of the gas is obtained. When the gas is no longer required the basket is raised out of the liquid, and the evolution of gas at once ceases. The funnel, T, serves for the introduction of the liquid, the tap, H, for drawing off the iron sulphate solution, which can be used with advantage for the preparation of fine iron colours. The apparatus should not be opened so long as sulphide of iron remains in the basket. =Sulphuric Acid= comes into the market in two different forms: oil acid; both are used in colour making. _Oil of Vitriol_, H₂SO₄ = 98, is a colourless, oily liquid of high specific gravity; it is generally tolerably pure, and contains, as a rule, only a small quantity of lead, the presence of which is indicated by a turbidity on largely diluting the acid. The amount of pure sulphuric acid in the liquid is practically determined by taking the specific gravity. The table indicates the relation between the specific gravity and the content of sulphuric acid.