Although the use of direct steam is very convenient for heating, since wooden vessels can be used and the heating quickly accomplished, it may be dispensed with, and the solutions heated in pans or even not at all. In this case the oxidation lasts considerably longer than when the liquid is heated. In the process recommended by Gentele 109 parts of yellow prussiate are used to 20 parts of green vitriol, each dissolved in much water. The precipitate is heated for a short time with 51 parts of nitric acid of 1·2285 specific gravity and 16 parts of sulphuric acid. The mixture is allowed to stand for several days to complete the oxidation before the precipitate is separated from the liquid. In Hochstätter’s method, solutions of 6 parts of yellow prussiate in 15 parts of water and of 6 parts of green vitriol in 5 parts of water are mixed. To the precipitate 24 parts of strong hydrochloric acid and 1 part of sulphuric acid are added, and then a solution of bleaching powder in 80 parts of water until the liquid smells distinctly of chlorine. Apart from the fact that this method does not employ equivalent quantities of iron salt and yellow prussiate, it is also to be regarded as unsuitable, since the addition of sulphuric acid produces calcium sulphate, which, being soluble with great difficulty, will be precipitated along with the blue, making the shade lighter. The addition of sulphuric acid should be omitted and only hydrochloric acid used, the calcium chloride formed by the decomposition of the bleaching powder being very soluble in water. Many formulæ given for the manufacture of Chinese blue are said to produce a pigment of special beauty. This, however, is incorrect. Chinese blue is a compound of fixed composition, which is always formed when equivalent quantities of the salts are used and the precipitate converted into the blue compound by one of the oxidising agents given. It is most simple to use nitric acid as the oxidising agent; for this purpose the presence of chlorine in the acid is harmless. When sufficient space and time are allowed, a special oxidising agent may be omitted and the oxidation of the white precipitate accomplished by atmospheric air. In working in this manner the precipitate is washed several times with water, and spread in a thin layer upon boards which are placed in rows one above the other. In a short time the mass turns blue on the surface, but the pale colour remains underneath much longer, since the access of oxygen to the interior is difficult. The boards must be left until the mass has a uniform colour throughout. In order that the paste shall not dry too thoroughly whilst on the boards it is desirable to make it very fluid. If the precipitate has been completely washed before the oxidation the product is finished after drying, but if a portion when completely dried shows a crystalline incrustation, salts are still present, which must be removed by careful washing.