[Footnote: Paper read before Section B, British Association, Aberdeen
meeting.]
By ALEXANDER WATT, F.I.C., F.C.S.
The late Dr. Mohr[1] of Bonn, advocated the use of a centrifugal machine
as a means of rapidly drying crystals and crystalline precipitates; but
although they are admirably adapted for that purpose, centrifugal
machines are seldom seen in our chemical laboratories.
[Footnote 1: "Lehrb. d. Chem. Analyt. Titrirmethode," 3d ed., 1870, p.
684.]
The neglect of this valuable addition to our laboratory apparatus is
probably owing to the inconvenience involved in driving the machine at a
high speed by means of the ordinary hand driving gear, especially when
the rotation has to be maintained for a considerable length of time. It
occurred to me, therefore, that by attaching the drum or basket of the
machine (or the rotating table of Mohr's apparatus) directly to the
spindle of an electro-motor, the difficulty of driving might be got over,
and at the same time a combination of great efficiency would result, as
the electro-motor, like the centrifugal machine, is most efficient when
run at a high speed. The apparatus shown in the sketch consists
essentially of a perforated basket, A, which is slipped on to a cone
attached to the spindle, S, of an electro-motor, and held in position by
the nut, D. The casing, B, with its removable cover, C, serves to receive
the liquid driven out of the substance being dried. A flat form of the
ordinary Siemens H armature, E, revolves between the poles, P, of the
electro-magnets, M, which are connected by means of the base plate, I.
The brass cross-bar, G, carries the top bearing of the spindle, S, and
prevents the magnet poles from being drawn together.
[Illustration]
From four to six cells of a bichromate battery or Faure secondary battery
furnish sufficient power to run the machine at a high speed. An apparatus
with a copper basket four inches in diameter has been found extremely
useful in the laboratory for drying such substances as granulated
sulphate of copper and sulphate of iron and ammonia, but more especially
for drying sugar, which when crystallized in very small crystals cannot
be readily separated from the sirupy mother-liquor by any of the usual
laboratory appliances. For drying substances which act on copper the
basket may be made of platinum or ebonite; in the latter case, owing to
the increased size of the perforations, it may be necessary to line the
basket with platinum wire gauze or perforated parchment paper.
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