alcohol (Fig. 8). After the bulb has been filled with the mercury, it
is placed in a beaker of water and the water is heated by a Bunsen
burner. As the water becomes warmer and warmer the level of the
mercury in the tube steadily rises until the water boils, when the
level remains stationary (Fig. 9). A scratch is made on the tube to
indicate the point to which the mercury rises when the bulb is placed
in boiling water, and this point is marked 212°. The tube is then
removed from the boiling water, and after cooling for a few minutes,
it is placed in a vessel containing finely chopped ice (Fig. 10). The
mercury column falls rapidly, but finally remains stationary, and at
this level another scratch is made on the tube and the point is marked
32°. 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° and 0°, and the interval between the points is
divided into 100 equal parts instead of into 180.

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

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