It is very desirable that those who do not prepare their emulsion by
boiling, but by prolonged digestion, should possess a regulator which
will keep the temperature at a given point. Such an apparatus would also
be very useful for warming the emulsion for the preparation of plates,
as then one would have no further occasion to pay attention to the
thermometer and gas stove. In the accompanying diagram a simple
contrivance is shown. The gas which feeds the stove passes through
a narrow glass tube, a b, into the wider tube, c d e, which is made
air-tight at e. This latter tube has an exit tube at f, by which the gas
is supplied to the gas stove. At e it is hermetically closed, and at its
deepest part it contains mercury, upon which a little sulphuric ether
floats in the hermetically-closed limb, e.g. Lastly, there is a minute
opening in the narrowest tube at i. The whole apparatus, or, at least,
the under part of it, is dipped into the water bath warmed by the gas
boiler. It acts thus: As the temperature rises the ethereal vapor in the
shorter limb expands and drives the mercury up the longer tube until
it closes the opening of the narrow tube, a b, and thereby impedes the
power of the stream of gas. Still, the Bunsen burner does not go out,
being always fed by the small opening, i, with sufficient gas to support
a small flame until the water bath has so far cooled as to leave the
opening at b free, when the burner again burns with a strong flame. By
removing the cork, c, from the tube the temperature of the water bath is
raised, while by pushing it in it is lowered. The apparatus never goes
wrong, and is very cheap. It was first made by Herr C. Braun, of Berlin.
[Illustration: FIG. 1.]