General Science | Page 4

mountain peak is covered with
cracked rock so insecure that a touch or step will dislodge the
fragments and start them down the mountain slope. The lower levels of
mountains are frequently buried several feet under débris which has
been formed in this way from higher peaks, and which has slowly
accumulated at the lower levels.
5. Temperature. When an object feels hot to the touch, we say that it
has a high temperature; when it feels cold to the touch, that it has a low
temperature; but we are not accurate judges of heat. Ice water seems
comparatively warm after eating ice cream, and yet we know that ice
water is by no means warm. A room may seem warm to a person who
has been walking in the cold air, while it may feel decidedly cold to

some one who has come from a warmer room. If the hand is cold,
lukewarm water feels hot, but if the hand has been in very hot water
and is then transferred to lukewarm water, the latter will seem cold. We
see that the sensation or feeling of warmth is not an accurate guide to
the temperature of a substance; and yet until 1592, one hundred years
after the discovery of America, people relied solely upon their
sensations for the measurement of temperature. Very hot substances
cannot be touched without injury, and hence inconvenience as well as
the necessity for accuracy led to the invention of the thermometer, an
instrument whose operation depends upon the fact that most substances
expand when heated and contract when cooled.
[Illustration: FIG. 8.--Making a thermometer.]
6. The Thermometer. The modern thermometer consists of a glass tube
at the lower end of which is a bulb filled with mercury or colored
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.
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° 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° F. and quickly cooled, it gives the fine cutting edge of
razors; if it is heated to about 500° F. and then cooled,