After bleaching, a certain quantity of calcium hypochlorite always remains in the pulp. It may be removed in the potcher itself, or when the pulp reaches the beater. In any case, the methods adopted are the same. (See p. 127.) In some mills it is the practice to free the bleached pulp from the products of the decomposition of the bleach by pressing the pulp in hydraulic presses, or by draining in vats provided with perforated bottoms. Or the pulp may be made to pass through a pulp-saver (see Fig. 52, p. 155.) Chlorine gas as a bleaching agent has been almost entirely superseded by the more manageable calcium hypochlorite. Its employment is open to the serious objection indicated before, viz. the liability to form difficultly removable chlorine compounds. Its use as a disintegrating agent has already been alluded to (see p. 102). A process has lately been invented (English patent, No. 11,333, 1884), by Prof. Lunge, which involves the use of acetic acid. The quantity required is, however, very small, as during the process of bleaching it becomes regenerated. The free lime in the bleaching powder solution should first be {115} nearly neutralised with a cheaper acid, such as hydrochloric or sulphuric, followed then by the addition of the acetic acid. This process, we are informed, gives excellent results with high-class material, such as the best cotton and linen rags; it is, however, not to be recommended for such materials as straw or esparto. For the bleaching of rags the process invented by Thompson (English patent, No. 595, 1883) has proved very effective. The method consists in saturating the material with a weak solution of bleaching powder, and then exposing them to the action of carbonic acid gas. In this way the bleaching action is made very rapid and effective. _Electrolytic Bleaching._—Various attempts have from time to time been made to bleach by means of electricity—that is to say, by means of the products of the electrolysis of solutions of alkaline chlorides and the chlorides of the alkaline earths. The only process which has been to any extent developed is that of M. Hermite. This process is based upon the electrolysis of a solution of magnesium chloride, this salt having been found to give the most economical results. The solution, at a strength of about 2·5 per cent. of the anhydrous salt (MgCl_{2}), is electrolysed until it contains the equivalent of about 3·0 grms. chlorine per litre. This solution is then run into the “poacher” containing the pulp to be bleached; a continuous stream is then kept up, the excess being removed by means of a drum-washer. This excess, which after being in contact with the pulp in the engine is more or less deprived of its bleaching properties, is then returned to the electrolysing vat, where it is again brought up to the normal strength.