The usual conception of heat is that it is a form of energy produced by
the vibratory motion of the minute particles or molecules of a body. All
bodies are assumed to be composed of these molecules, which are held
together by mutual cohesion and yet are in a state of continual
vibration. The hotter a body or the more heat added to it, the more
vigorous will be the vibrations of the molecules.
As is well known, the effect of heat on a body may be to change its
temperature, its volume, or its state, that is, from solid to liquid or
from liquid to gaseous. Where water is melted from ice and evaporated
into steam, the various changes are admirably described in the lecture
by Mr. Babcock on "The Theory of Steam Making", given in the next
chapter.
The change in temperature of a body is ordinarily measured by
thermometers, though for very high temperatures so-called pyrometers are
used. The latter are dealt with under the heading "High Temperature
Measurements" at the end of this chapter.
[Illustration: Fig. 11]
By reason of the uniform expansion of mercury and its great
sensitiveness to heat, it is the fluid most commonly used in the
construction of thermometers. In all thermometers the freezing point and
the boiling point of water, under mean or average atmospheric pressure
at sea level, are assumed as two fixed points, but the division of the
scale between these two points varies in different countries. The
freezing point is determined by the use of melting ice and for this
reason is often called the melting point. There are in use three
thermometer scales known as the Fahrenheit, the Centigrade or Celsius,
and the Réaumur. As shown in Fig. 11, in the Fahrenheit scale, the space
between the two fixed points is divided into 180 parts; the boiling
point is marked 212, and the freezing point is marked 32, and zero is a
temperature which, at the time this thermometer was invented, was
incorrectly imagined to be the lowest temperature attainable. In the
centigrade and the Réaumur scales, the distance between the two fixed
points is divided into 100 and 80 parts, respectively. In each of these
two scales the freezing point is marked zero, and the boiling point is
marked 100 in the centigrade and 80 in the Réaumur. Each of the 180, 100
or 80 divisions in the respective thermometers is called a degree.
Table 3 and appended formulae are useful for converting from one scale
to another.
In the United States the bulbs of high-grade thermometers are usually
made of either Jena 58^{III} borosilicate thermometer glass or Jena
16^{III} glass, the stems being made of ordinary glass. The Jena
16^{III} glass is not suitable for use at temperatures much above 850
degrees Fahrenheit and the harder Jena 59^{III} should be used in
thermometers for temperatures higher than this.
Below the boiling point, the hydrogen-gas thermometer is the almost
universal standard with which mercurial thermometers may be compared,
while above this point the nitrogen-gas thermometer is used. In both of
these standards the change in temperature is measured by the change in
pressure of a constant volume of the gas.
In graduating a mercurial thermometer for the Fahrenheit scale,
ordinarily a degree is represented as 1/180 part of the volume of the
stem between the readings at the melting point of ice and the boiling
point of water. For temperatures above the latter, the scale is extended
in degrees of the same volume. For very accurate work, however, the
thermometer may be graduated to read true-gas-scale temperatures by
comparing it with the gas thermometer and marking the temperatures at 25
or 50 degree intervals. Each degree is then 1/25 or 1/50 of the volume
of the stem in each interval.
Every thermometer, especially if intended for use above the boiling
point, should be suitably annealed before it is used. If this is not
done, the true melting point and also the "fundamental interval", that
is, the interval between the melting and the boiling points, may change
considerably. After continued use at the higher temperatures also, the
melting point will change, so that the thermometer must be calibrated
occasionally to insure accurate readings.