(Take of pure bichromate of potash, one hundred and twenty grains; distilled water, one pint. Dissolve.) Of the bichromate of potash solution, one hundred measures contain 14.75 (or 1/10 of an equivalent) in grains of that salt. It is used to determine the amount of a protosalt of iron present in a liquid. When the bichromate is added to a solution of a protosalt of iron in hydrochloric acid, the proto- is converted into per-salt, and by ascertaining the amount of the solution necessary to complete this change, the quantity of protosalt present can be estimated. The nature of the decompositions which occur may be represented by the following formula, KO, 2 CrO3 + 6 FeO + 4 H Cl=3 (Fe2 O3) + Cr2 Cl3 + KCl + 4 HO. Two equivalents of chromic acid (2 CrO3), present in the bichromate, yield three equivalents of oxygen, which convert six equivalents of protoxide, 6 (FeO), into three of peroxide 3 (Fe2 O3). Consequently, 147.5, the equivalent in grains of bichromate of potash, is capable of converting, and will therefore represent, six equivalents in grains of protoxide of iron; and one hundred measures of the volumetric solution, containing 14.75 grains of the bichromate, will represent 16.8 or 1/10 of six equivalents of iron. When all the iron is converted into a persalt, a drop of the solution will no longer strike a blue colour with ferridcyanide of potassium.