(Take of hyposulphite of soda, in crystals, two hundred and sixty grains; distilled water, a sufficiency.) Dissolve the hyposulphite of soda in one pint of the water, and drop the solution cautiously from the volumetric tube into one hundred measures of the volumetric solution of iodine, until the brown colour of the iodine is just discharged. Note the number of measures (n) which have been used to produce this effect; and having then taken sixteen fluid ounces of the same solution augment this quantity by the addition of distilled water until it amounts to 1600/N fluid ounces. If for example, n = 96, the sixteen ounces of the solution of the hyposulphite should be diluted with distilled water so as to become 1600/96 = 16.66 fluid ounces. One hundred measures of the solution of hyposulphite of soda contain 24.8 grains of the crystallized hyposulphite. It is employed for estimating free iodine. When the solution of hyposulphite is added to a liquid containing free iodine a decomposition takes place, which may be represented by the formula, 2 (NaO, S2 O3) + I = NaO, S4 O5 + Na I, iodide of sodium and tetrathronate of soda being formed, the solutions of which are devoid of colour. Hence, to decolorize one equivalent in grains of iodine, two equivalents in grains of the hyposulphite are required. Now one hundred measures of the volumetric solution contain 24.8 or 1/10 of two equivalents in grains of the hyposulphite of soda, and 27 will therefore represent 12.7 or 1/10 of one equivalent in grains of iodine.