4. Page 201

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THERMAL TREATMENTS.
201

indicated by it, at a white-heat, comparable with the degrees of
the ordinary thermometer? The author believes that the laws
of Boyle and of Charles will probably hold good at the high
temperatures of ordinary furnaces; and, further, the evidence
as to temperature indicated by the air-thermometer does not rest
upon the expansion of a single gas, as the porcelain bulb may be
filled with nitrogen, oxygen, or carbonic anhydride. The question
as to the degree of confidence which may be reposed in the
numerical values of high temperatures is, however, so important,
that the author would refer to the following experiment of Carl
Barus, who has devoted years of patient work to pyrometric inves-
tigations. Fig. 45a shows the arrangement adopted by him for
comparing directly the air-thermometer with the thermo-junction.*
The latter is inserted in a tubulure extending to the centre of the

FIG. 45a.

bulb e: and the disposition of the various parts of the apparatus
is as follows. The walls of a cylindrical furnace B B are covered
with a hemispherical dome A A. The furnace is heated by gas,
introduced through the burners G G, H H; compressed air enter-
ing by the inner tubes g g and h h. The inlets for the gas are
c c’. The furnace can be heated to a high temperature with ease;
but in order to equalise the heat, Barus employs an internal
globular “muffle,” E C D F. It consists of two hemispheres of
fire-clay, provided with lateral tubes, which pass through the walls
of the furnace. The two hemispheres are held together by the
iron collars N N, N’ N’. The outer edges of these collars P P’ are
flanged, and fit into the grooves of two friction rollers, Q Q’, of
which R R’ are the respective axes. There are adjusting screws
at V V’, u u‘, t t’. The muffle is rotated by a belt pulley screwed
on to the flange P’. The air-thermometer is shown in position

* Bulletin United States Geological Survey, No. 54, Washington, 1889.