The operation of boiling substances under pressure with more or less dilute sulphuric or sulphurous acid forms a necessary stage of several important manufactures, such as the production of paper from wood, the extraction of sugar, etc. A serious difficulty attending this process arises from the destructive action of the acid upon the boiler or chamber in which the operation is carried on, and as this vessel, which is generally of large dimensions, is exposed to considerable pressures, it is necessarily constructed of iron or some other sufficiently resisting metal. An ingenious method of avoiding this difficulty has been devised, we believe in Germany, and has been put into practice with a certain amount of success. It consists in lining the iron boiler with a covering of lead, caused by fusion to unite firmly to the walls of the boiler, and thus to protect it from the action of the acid. No trouble, it is stated, is found to arise from the difference in expansion of the two metals, which, moreover, adhere fairly well; but, on the other hand, we believe it does actually occur that the repairs to this lead lining are numerous, tedious, and costly of execution, so that the system can scarcely be regarded as meeting the requirements of the manufacturer. It is to secure all the advantages possessed by a lead-lined vessel, without the drawback of frequent and expensive repairs, that the digester, of which we annex illustrations, has been devised by Mr. George Knowles, of Billiter House, Billiter Street. It consists of a closed iron cylindrical vessel suitable for boiling under pressure, and containing a second vessel open at the top, and of such a diameter as to leave an annular space between it and the walls of the outer shell. This inner receiver, which may be made of lead, glass, pottery, or any other suitable material, contains the substance to be treated and the sulphurous acid or other solution in which it is to be boiled. The annular space between the two vessels is filled with water to the same level as the solution in the receiver, and the latter is provided with suitable pipes or coils, in which steam is caused to circulate for the purpose of raising the solution of the desired temperature, and effecting the digesting process. At the same time any steam generated collects in the upper part of the boiler, and maintains an equal pressure within the whole apparatus. Figs. 1 to 3 show the arrangement clearly. Within the boiler, a, is placed the receiver, b, of pottery, lead, or other material, leaving an annular space between it and the boiler; this space is filled with water. The receiver, b, is furnished with a series of pipes, in which steam or hot water circulates, to heat the charge to the desired temperature. These pipes may be arranged either in coils, as shown at d, Fig. 1, or vertically at d, Fig. 3. The latter are provided with inner return pipes, so that any condensed water accumulating at the bottom may be forced up the inner pipes by the steam pressure and escape at the top. The vessel is charged through the manhole, e, and the hopper, c, provided with a perforated cover, and is discharged at the bottom by the valve, f, shown in Figs. 2 and 3. The upper part of the boiler serves as a steam dome, and the pressure on the liquid in the receiver and on the water in the annular space is thereby maintained uniform. The necessary fittings for showing the pressure in the vessel, water level indicator, safety valve, cocks for testing solutions, etc., are of course added to the apparatus, but are not indicated in the drawing. The arrangement appears to us to possess considerable merit, and we shall refer to it again on another occasion, after experiments have been made to test its efficiency.--Engineering.

IMPROVED DOUBLE BOILER.

IMPROVED DOUBLE BOILER.