It has been seen that a certain weight of water always has the same
volume under the same conditions. The most important of these conditions
is the heat or cold to which it is exposed. Water which has stood for
some time in a warm room becomes less in volume, or =contracts=, if it
is taken into a cool place; while its volume increases, or it =expands=,
if it is made hot. The same thing is true of quicksilver, of spirit, and
of liquids in general. A =thermometer= is simply a small flask—the
bulb—with a long and narrow neck—the tube—filled with as much mercury or
spirit as will rise a short distance into the neck. If the liquid in the
bulb is warmed, its volume is increased and it overflows into the tube,
increasing the height of the column of liquid in the tube. If, on the
other hand, the liquid in the bulb is cooled, its volume is diminished;
and, as it shrinks, the column of liquid in the tube flows back into the
bulb, and the level of the top of the column is lowered.
If a mark is made on the tube, or on a scale fixed to it, at the point
which the liquid reaches when the bulb is placed in boiling water; and
another mark at the point to which it sinks when the bulb is in melting
ice; and the space between the two marks is divided into 180 equal
parts, each of these parts is what is called a “degree” in the
thermometers ordinarily used in this country (called Fahrenheit). And if
the boiling-point is counted as 212° the freezing-point must be 32° (212
- 32 = 180). With the same amount of heat the fluid in the tube always
stands at the same degree, and hence the instrument measures
=temperature=.
That hot water is lighter than cold is easily seen when a bath is filled
from two taps, one of hot and one of cold water, which run at the same
time. Unless care is taken to stir the water, the top of the bath will
be very much hotter than the bottom. Thus, an imperial pint of water
weighs a pound and a quarter only at a certain temperature or degree of
warmth, namely at 62°; if it is made hotter its volume increases, and
therefore its specific gravity diminishes.
It was for this reason that in § 22 the weight of the same volume of
water was said to be constant =under the same conditions=; and, of
course, the same qualification must be borne in mind when we speak of
the weight of a cubic inch of water being about 252 and a half grains.
Its weight is in fact 252·45 grains only when Fahrenheit’s thermometer
stands at 62°—but as this is the temperature of ordinary mild weather,
and the expansion or contraction of water for a degree about this
temperature amounts to less than 1/3000th of its volume, the weight of a
cubic inch may for all practical purposes be taken as 252 and a half
grains.