t is marked
32 deg.. The space between these two points, which represent the
temperatures of boiling water and of melting ice, is divided into 180
equal parts called degrees. The thermometer in use in the United
States is marked in this way and is called the Fahrenheit thermometer
after its designer. Before the degrees are etched on the thermometer
the open end of the tube is sealed.

[Illustration: FIG. 9.--Determining one of the fixed points of a
thermometer.]

The Centigrade thermometer, in use in foreign countries and in all
scientific work, is similar to the Fahrenheit except that the fixed
points are marked 100 deg. and 0 deg., and the interval between the points is
divided into 100 equal parts instead of into 180.

_The boiling point of water is 212 deg. F. or 100 deg. C_.

_The melting point of ice is 32 deg. F. or 0 deg. C_.

Glass thermometers of the above type are the ones most generally used,
but there are many different types for special purposes.

[Illustration: FIG. 10.--Determining the lower fixed point of a
thermometer.]

7. Some Uses of a Thermometer. One of the chief values of a
thermometer is the service it has rendered to medicine. If a
thermometer is held for a few minutes under the tongue of a normal,
healthy person, the mercury will rise to about 98.4 deg. F. If the
temperature of the body registers several degrees above or below this
point, a physician should be consulted immediately. The temperature of
the body is a trustworthy indicator of general physical condition;
hence in all hospitals the temperature of patients is carefully taken
at stated intervals.

Commercially, temperature readings are extremely important. In sugar
refineries the temperature of the heated liquids is observed most
carefully, since a difference in temperature, however slight, affects
not only the general appearance of sugars and sirups, but the quality
as well. The many varieties of steel likewise show the influence which
heat may have on the nature of a substance. By observation and tedious
experimentation it has been found that if hardened steel is heated to
about 450 deg. F. and quickly cooled, it gives the fine cutting edge of
razors; if it is heated to about 500 deg. F. and then cooled, the metal is
much coarser and is suitable for shears and farm implements; while if
it is heated but 50 deg. F. higher, that is, to 550 deg. F., it gives the fine
elastic steel of watch springs.

[Illustration: FIG. 11.-