As the various bisulphite processes differ chiefly in their working details, the essential feature of the employment of a solution of sulphurous acid, combined with a certain amount of base, being common to them all, it will not be necessary to do more than examine one or two typical cases. We shall select those of Ekman and Partington, the latter being based upon Francke’s patent, with certain modifications and additions. (1) The Chemicals employed.—In both cases sulphur is used as the source of the sulphurous acid. The Ekman process starts from magnesite, a native carbonate of magnesia. This is converted by calcination into magnesia, and in this state is introduced into lead-lined towers, through which is passed sulphurous acid gas, obtained by burning sulphur in suitable furnaces, while a stream of water trickles down from the top. The supply of the gas being properly adjusted, a continuous production of a solution of magnesium bisulphite of uniform strength is obtained. To secure this uniformity, constant attention is necessary, and further, the loss of sulphurous acid by oxidation to sulphuric acid, and by diffusion, is, in consequence of its distribution over so large a surface, {73} and with free exposure to the air, necessarily very large. It is unnecessary, however, to point out that the preparation of the magnesium bisulphite is capable of very considerable modification; in fact, we understand that numerous improvements in this direction have already been introduced by the proprietors of the process. Partington’s method, in addition to the advantage of starting from lime, the cheapest of all basic substances, is free from the disadvantages referred to as attending the use of a tower. It consists in passing the sulphurous acid gas through a series of three vessels connected together with pipes. Into these vessels a charge of milk of lime is introduced, and the two first being closed air-tight, the gas is passed in. The third remains open to the air, and from it escapes a continuous stream of nitrogen, resulting from the removal of the oxygen by the burning sulphur, from the stream of air which is blown through the sulphur retort at a pressure of 5 lb. to the square inch. The expenditure for chemicals is under 1_l._ per ton, whereas, by the Ekman process, it has been variously estimated at from 2_l._ to 3_l._ 10_s._ per ton. Provided therefore, that as good pulp can be obtained by Partington as by Ekman, the former offers considerable advantages both from point of view of simplicity and economy. Doubtless the cost of chemicals could be somewhat reduced in both systems by the adoption of a cheaper form of sulphur, such as pyrites.