Take of Nitrate of Baryta 300 grains;--powder and dissolve by the aid of heat in three ounces of water. Then throw in by degrees, with constant stirring, crystallized Sulphate of Iron, _powdered_, 320 grains. Continue to stir for about five or ten minutes. Allow to cool, and filter from the white deposit, which is the insoluble Sulphate of Baryta. In place of Nitrate of Baryta, the Nitrate of Lead may be used (Sulphate of Lead being an insoluble salt), but the quantity required will be different. The atomic weights of Nitrate of Baryta and Nitrate of Lead are as 131 to 166; consequently 300 grains of the former are equivalent to 380 grains of the latter. THE FIXING SOLUTION. Cyanide of Potassium 2 to 12 grains. Common Water 1 fluid ounce. Cyanide of Potassium is usually preferred to Hyposulphite of Soda for fixing direct Positives; it is less liable to injure the purity of the white colour. The percentage of _Carbonate of Potash_ in commercial Cyanide of Potassium is so variable that no exact directions can be given for the formula. It is best however to use it rather dilute--of such a strength that the plate is cleared gradually in from half a minute to a minute. The solution of Cyanide of Potassium decomposes slowly on keeping, but it will usually retain its solvent power for several weeks. In order to escape inconvenience from the pungent odour evolved by this salt, many employ a vertical Bath to hold the solution; but in that case the plates must be carefully washed before fixing, as the Iron salts hasten the decomposition of the Cyanide. THE WHITENING SOLUTION. Bichloride of Mercury 30 grains. Distilled Water 1 fluid ounce. By a gentle application of heat the corrosive sublimate dissolves and forms a solution as nearly as possible saturated at common temperatures. The addition of a portion of Muriatic Acid enables the water to take up a larger quantity of Bichloride; but this concentrated solution, at the same time that it whitens more quickly than the other, is apt to act unequally upon different parts of the image. Before applying the Bichloride, the image is to be fixed and the plate well washed. Either the Protosulphate of Iron or the Pyrogallic Acid with Acetic (p. 223) may be used for the development; but the whitening process is more rapid and uniform in the latter case. SECTION II. _Formulæ, etc., for Negative Solutions._[34] [34] The same Collodion and Nitrate Bath may be used both for Positives and Negatives if required; but there are a few minor points of difference which are included in the following remarks. THE COLLODION. FORMULA No. 1. Purified Ether, sp. gr. ·720 5 fluid drachms. Purified Alcohol, sp. gr. ·825 3 fluid drachms. Soluble Pyroxyline 4 to 8 grains. Pure Iodide of Cadmium or Ammonium 4 to 5 grains. FORMULA No. 2.