With a view to removing the difficulties attending the use of soda, various processes have from time to time been proposed, in which the hydrolytic agent is a solution of sulphurous acid in water, either alone or in combination with a base. These processes are comprised under group B. {65} Several of them have lately acquired great commercial and industrial importance, and a clear understanding of them is therefore essential to the student of paper-making. Though applicable to lignified fibres generally, they have only been applied up to the present time to wood, and it is in this connection, therefore, that we propose to consider them. In order to bring out more clearly the fundamental differences underlying the two classes of processes, it will be well to consider, somewhat in detail, these processes, which, although they have now a merely historical interest, will assist the student in thoroughly grasping the rationale of the modern and improved methods. Water Process.—Wood is to a certain extent resolved by treatment with water at a high temperature, the degree of disintegration being, within certain limits, proportional to the temperature at which it is digested. The volatile products of the resolution are chiefly acids of the acetic series, furfurol and terpenes. The non-volatile soluble products are for the most part acids. This process was studied by Fry, who, in 1866, made it the subject of a patent. He found that under the most favourable conditions of treatment, the yield of brown pulp amounted to 70 per cent. of the wood employed. The objections to the process were sufficiently obvious: the pulp produced was of low quality, and the soluble products acted powerfully on the iron digesters employed. The chemical causes of its failure lie in the prominence of the conditions which determine re-combinations of the products of resolution with one another, viz. oxidation, high temperature, and the presence of acids. It is worthy of note that in one experiment the amount of acids produced from 100 parts wood were equivalent to 3·4 parts of soda (Na_{2}O). This was by far the most serious defect in the process, and it eventually caused its abandonment in favour of the older method of treatment with caustic alkali.[9] [Footnote 9: See Journ. Soc. of Arts., February 1883, p. 223.]