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.
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