or Reboulleau's Blue, is prepared by fusing together equal weights of ordinary arseniate of protoxide of copper and arseniate of potash, and adding one-fifth its weight of nitre to the fused mass. The result is, so to speak, a sort of blue Scheele's green, into which latter colour it soon passes when rubbed with oil. 146. _Cotton Seed Blue._ Cotton seed oil is bleached by treatment with either carbonate of soda or caustic lime. In both cases, a considerable residue is left after drawing off the bleached oil. This residue is treated with sulphuric acid, and distilled at a high temperature, when there is left a compact mass of a deep greenish-blue colour. On further treatment of this mass with strong sulphuric acid, the green tint disappears, and a very intense pure blue colour is produced. The blue mass is a mixture of the coloured substance with some sulphuric acid, sulphate of soda, and fats. The two former may be removed by washing with water; the latter by treatment with naptha. Alcohol now dissolves the blue colour, and water precipitates it from the solution chemically pure. This blue has not been introduced as a pigment; and of its permanence, and other attributes, we know nothing. 147. _Gold Blue._ Gold purple, under the name of Purple of Cassius, was once very well known: a like compound of tin and gold may be made to yield a blue. Resembling indigo, the colour is not remarkably brilliant, and, unless several precautions are carefully observed, is rather violet than blue. When obtained, the colour must be quickly washed by decantation, or it changes first to violet and then to purple. Its costliness, lack of brightness, and tendency to redden, are against its employment on the palette. In enamelling it would doubtless preserve its colour, and in exceptional cases might be useful. 148. _Iodine Blue._ It is curious that iodine, which gives a yellow with lead, should also afford a blue with the same metal. When a solution of iodine in aqueous soda (carbonate of soda is not so good) is added to nitrate or acetate of lead-oxide, a transient violet-red precipitate falls, which decomposes spontaneously under water, yielding iodine and a beautiful blue powder. The colour, however, is exceedingly fugitive, even the carbonic acid of the air separating iodine from it and forming a lead salt. Bearing in mind the scarlet iodide of mercury, iodine is capable of furnishing the three primary colours, distinguished alike by their brilliancy and fugacity. 149. _Iridium Blue._ The rare metal iridium affords a blue which is a mixture of the oxide and the sesquioxide. But being slightly soluble in water and decolourised by sulphuretted hydrogen, it would not, other considerations apart, be an acquisition. 150. _Manganese Blue._