The bleached half-stuff as it leaves the steeping chests usually contains an excess of bleaching liquor, which can be removed in two ways, viz. by washing or by decomposition with an “antichlor.” The first method has the advantage of not only removing the bleach, but of also eliminating the chloride of calcium, partly existing ready formed, and also that resulting from the decomposition of the calcium hypochlorite originally present in the bleach. On the other hand, this method takes some time, and consumes a large amount of water, which in some mills is a matter of considerable importance. For this purpose, many beaters are provided with one or more drum washers (see Fig. 36). An additional objection to this method lies in the fact that a certain quantity of fibre passes through the meshes of the wire-cloth covering the washers, and is thus lost. The more usual plan is to remove the bleach by decomposing it with an “antichlor.” The substance generally employed for this purpose is sodium hyposulphite, or thiosulphate as it is now called, which, in presence of calcium hypochlorite, is oxidized to sodium sulphate, the latter being reduced to calcium chloride. Double decomposition then takes place between these salts, with the formation of calcium sulphate and sodium chloride. The reactions which take place may be expressed by the following equation:— 2 (Ca(ClO)_{2}) + Na_{2}S_{2}O_{3} + H_{2}O = 2 CaSO_{4} + 2 HCl + 2 NaCl. Calcium Sodium Water. Calcium Hydrochloric Sodium hypochlorite. thiosulphate. sulphate. acid. chloride. {128} The above decomposition does not accurately represent the action of bleach upon sodium thiosulphate. If the solutions employed are very dilute, the decomposition may take place in another direction, viz.:— Ca(ClO)_{2} + 4 Na_{2}S_{2}O_{3} + H_{2}O = 2 Na_{2}S_{4}O_{6} + 2 NaCl Calcium Sodium Water. Sodium Sodium hypochlorite. thiosulphate. tetrathionate. chloride. + 2 NaOH + CaO. Caustic soda. Lime. At the particular degree of dilution which occurs in a beater, the bleach is decomposed almost entirely according to the first of the equations; from which, on calculating, it will be seen that 158 parts of sodium thiosulphate are equivalent to 286 parts of calcium hypochlorite. As commercial sodium thiosulphate contains 36·3 per cent. of water, and bleaching powder 70 per cent. of calcium hypochlorite, on the basis of 35 per cent. available chlorine it follows that 248 parts of the former are required to neutralize 409 parts of the latter.