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. The platinum heat carrier, sometimes at a temperature of
2,500 deg. to 2,800 deg. F., is thereby brought into more rapid and forcible
contact with the water, steam or water in the spherical condition is
washed away from its surface, and by cooling it more rapidly, the
duration of the observation is lessened, and errors due to transmission
of heat through the walls of the instrument are diminished. The upper
part of the agitator stem is of hard rubber, and the brass portion,
which terminates at the under side of the cover when the agitator is in
its lowest position, suspended by the shoulder at the upper end, need