The cellulosic products of the various processes present certain slight differences, but so far as has been investigated, not such as to constitute a distinct advantage for one or {75} another. One feature of these processes, to which but little attention has hitherto been paid, is that of the utilisation of their bye-products. These bodies are, as already indicated, obtained in a condition not materially different from that of the parent substance, the non-cellulose constituents of the wood. One important property of these bodies, and one capable of considerable application, has been made the basis of a patent by the authors. When a solution of gelatin is added to the liquors in which wood has been treated by any of the bisulphite or sulphurous acid processes, a body is precipitated in the form of a caoutchouc-like mass which can be dissolved in neutral sodium sulphite solution, or in weakly alkaline solutions, and can be reprecipitated on addition of alum to the solution. The substance consists of a compound of the gelatin with the non-cellulose constituents resembling, but not identical with, as has been supposed, the precipitate obtained by adding tannic acid to a solution of gelatin. The solution of this substance in sodium sulphite can be applied to many of the purposes to which gelatin itself is ordinarily applied. A serious objection to all processes involving the use of sulphurous acid or acid sulphite, and one which has to some extent deterred paper-makers from adopting them, is the necessity of using a boiler lined with lead. In addition to the fact that such boilers are heavy and costly, they are objectionable on account of the difficulty and expense of keeping them in thorough repair. This difficulty arises from the fact that, owing to the different co-efficients of expansion of lead and iron, the lead lining originally placed in contact with the iron or steel shell of the boiler, becoming detached on heating, swells out in various places, which are thus rendered weaker and liable to fracture. This is especially the case if, in the course of lining the boiler, any air has been allowed to remain between the two linings. Various expedients have been proposed to obviate these defects but with only partial success. Graham (Eng. Pat. No. 5367, 1883) coats the plate of his boiler by pouring molten lead upon the perfectly clean surface which has been {76} previously prepared with zinc chloride. Sometimes minute holes are drilled in the iron plate to allow the escape of air.