The agitator is an important part of the apparatus. Its object, in
this instrument, is twofold. _First_, it serves to produce a uniform
temperature throughout the body of water in the instrument; and
_secondly_, it answers as a support to the heat-carrier of platinum or
other metal, often intensely hot, which would injure or destroy the
delicate metal of the bottom if allowed to fall on it. For this second
purpose, no spiral revolving agitator, such as that commended by
Berthelot, would suffice. The best form is such as I have shown in Fig.
1. A concave disk of sheet-brass, made to conform to the shape of the
bottom of the cell, with a narrow rim turned up all around, of about
0.02 inch thickness, is liberally perforated with holes to lighten it,
and to give free passage to water. The concave form causes the streams
of water, produced by slightly raising and lowering the agitator, to
take a radial direction downward or upward, so as to cross each other
and promote rapid mixing. By a slight modification small vanes might be
turned outward from the surface of the metal, which would produce mixing
currents if the agitator were given a slight reciprocatory revolving
motion, thus avoiding the alternate withdrawal and re-immersion of any
part of the stem so strongly deprecated by Berthelot; but for several
reasons I think an up and down motion of the agitator desirable in this
instrument.
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