The next processes in the order of the development of our subject, but not chronologically, are those involving the use of reducing acids, i.e. those which combine with their hydrolytic function a de-oxidising or anti-oxidising action. Of these the only one which has been practically investigated is sulphurous acid. A full account of the earlier discoveries in regard to the use of this reagent, for the special purpose under consideration, will be found in the _Papier Zeitung_, 1884, No. 51, p. 1927; this should be consulted by those who are interested in the history of a subject which has now grown to be of the first importance. The first patented process claiming the use of sulphurous acid, in the form of its aqueous solution, for the purpose of disintegrating wood, was that of Tilghmann (1866). So far as is known, however, the results obtained by the inventor with the acid alone were such as to lead him to abandon it for solutions containing a certain proportion of base. The reasons, doubtless, were the greater facility of preparation and simplicity of application of such solutions. More recently Professor Pictet, of Geneva, on the basis of the experience acquired by him in the preparation and employment of sulphurous acid in his patent freezing processes, has revived and perfected a process which in other hands had proved impracticable. The method and apparatus employed will be found fully described in the author’s specification. The process consists in digesting the wood in closed vessels with an aqueous solution of the acid (10–12 per cent. SO_{2}) at 176°–194° F. (80°–90° C.), and under the corresponding pressure (60–75 lb. per square inch). A complete disintegration of pine wood is effected in about twenty-four hours’ treatment. The yield of pulp is, we are informed, from 40–50 per cent. of the wood (_Pinus sylvestris_); it is also stated to be readily bleached {71} with a moderate amount of bleaching powder, and to yield a pulp of the highest quality. Further, the recovery of the sulphurous acid, for which special apparatus is provided, is practically complete (95–98 per cent.). In view of the probable utilisation of the bye-products, it is interesting to observe that they are obtained in a form in which they can very readily be treated.