_Pyrometers filled with mercury_ are constructed for ascertaining
temperatures from 360 deg. to 750 deg. C.

[Illustration: Fig. 4.--Mounting on horizontal pipes by thread on the
tube.]

[Illustration: Fig. 5.--Mounting by means of a clasp in reservoirs.]


APPLICATION OF THE PYROMETER IN BONE BLACK FURNACES.

The temperature necessary for the complete carbonization of the organic
substances of animal charcoal is from 430 deg. to 500 deg. C. In order to
transmit this temperature from the cylinder to the charcoal it is
indispensable that the air surrounding the cylinder be heated to 480 deg.
to 550 deg.. If the heating of the animal black exceeds 500 deg. the product
hardens, diminishes in volume, and loses its porosity. There are two
methods of ascertaining the temperature of the red-hot bone black by
means of the pyrometer: First, by inserting the tube of the instrument
into the black. (Fig. 6, a.) Second, by finding the temperature of the
hot gases in the furnaces (Fig. 6, b.). In the first case, the plunge
tube should be of sufficient length to allow its extremity to penetrate
to the very bottom layer of the red-hot black. This mode of direct
control of the temperature of the black is only employed for
ascertaining the work accomplished by the furnace, that is to say, the
ratio existing between the temperature of the hot air surrounding the
cylinder and the black itself. This calculation being effected, it is
useless to note the differences of temperature which arise in the spaces
between the cylinders of which the furnace is composed.