er. It is also used in
the fitting up of water and paraffin baths, and for many other purposes.

[Illustration: FIG. 115.--Reichert's thermo-regulator.]

Of the many forms and varieties of thermo-regulator (other than
electrical), two only are of sufficiently general use to need mention.
In one of these the flow of gas to the gas-jet is controlled by the
expansion or contraction of mercury within a glass bulb; in the other,
by alterations in the position of the walls of a metallic capsule
containing a fluid, the boiling-point of which corresponds to the
temperature at which the incubator is intended to act. They are:

(a) _Reichert's_ (Fig. 115), consists of a bulb containing mercury
which is to be suspended in the medium, whether air or water, the
temperature of which it is desired to regulate. Gas enters at A, and
passes out to the jet by B. As the temperature rises the mercury expands
and cuts off the main gas supply. As the temperature falls the mercury
contracts and reopens the narrow tube C. By means of a thumbscrew D
(which mechanically raises or lowers the column of mercury irrespective
of the temperature) and the aid of a thermometer the apparatus can be
set to keep the incubator at any desired temperature. With this form a
special gas burner is required, with separate supply of gas to a pilot
jet at the side.

(b) _Hearson's capsule regulator_ consists of a metal capsule
hermetically sealed and filled with a liquid which boils at the required
temperature, this is adjusted in the interior of the incubator. Soldered
to the upper side of the capsule is a thick piece of metal having a
central cup to receive the lower end of a rigid rod, through which the
movements of the walls of the capsule are transmitted to the gas valve
fixed outside the incubator.

The gas valve or governor is shown in figure 116. A is the inlet for
gas, C the outlet to burner heating the water jacket, B D a lever
pivoted to standards at G, and acted upon by the capsule, through the
rigid rod which enters the socket below the screw P.

[Illustration: FIG. 116.--Capsule thermo-regulator.]

The construction of the valve is such that, whenever the short arm of
the lever B D presses on the disc below the end B, the main supply of
gas is entirely cut off. At such times, however, a very small portion of
gas passes from A to C, through an aperture inside the valve, the size
of which aperture can be adjusted by the screw needle S, hence the ga