All substances produce in us, when we touch them, the sensation of
heat or of cold. The degree of heat of any substance is called its
temperature; and the temperature varies from time to time, according
to circumstances. Boiling water, for example, contains so much heat
that it scalds the skin; but, when removed from the fire, the water
gradually becomes less and less warm, until at last it contains so
little heat that it cools the hand instead of scalding it.
Our feelings do not always give us true information about the
temperature of the bodies by which we are surrounded. A person comes
into a warm room from the open air on a cold day, and exclaims, “How
warm it is here!” Another person enters the same room from one still
warmer, and cries, “How cold it is here!” The first person gains heat,
and therefore calls the room warm; the second loses heat, and calls it
cold; while, in reality, the air of the room, all the while, is at the
same degree of temperature.
A nurse prepares water for a child’s bath, and measures the heat by
the feeling of her hand; but she learns from the quick and sudden cry
of the child, when placed in the bath, that what seemed warm to her is
cold to the child.
These examples show that our sensations are not always a true test of
temperature; and thus, if we wish to measure heat and cold accurately,
we must have some instrument made for the purpose. Such an instrument
we have: it is called a thermometer, a word which means heat-measurer.
It was long ago noticed that bodies expand, or swell out, when they are
heated; and that they contract, or shrink into less bulk, when they are
cooled. This observation led to the construction of the thermometer;
for to measure the expansion or contraction of a substance is the same
as to measure the quantity of heat that has produced this effect.
The thermometer consists of a small glass tube of very fine bore and
ending in a hollow bulb. The bulb is entirely filled with a fluid
metal, called mercury, or quicksilver, which rises or falls in the tube
according as it is heated or cooled. The space in the tube above the
mercury is empty: it does not even contain air; for in preparing the
instrument, sufficient heat is applied to the bulb to make the fluid,
by expansion, fill the whole tube, which is then closely sealed at the
upper end by melting the glass: the mercury sinks as it cools, and
leaves the space empty.
The tube is fastened to a metal or wooden plate, marked so as to
measure the expansion of the mercury. This is called the scale, and the
divisions upon it are called degrees. The degrees are counted upwards
from the bottom, and are expressed by placing a small circle over the
number. There are three different scales in use; that commonly used
in this country is called Fahrenheit’s scale, from the name of the
inventor.
When the tube of the thermometer is immersed in melting ice or freezing
water, the upper surface of the mercury stands always at the same
point, which is called the _freezing point of water_. On Fahrenheit’s
thermometer this point is marked 32°. Again, when the bulb is held in
the steam of boiling water, the mercury rises to a fixed point, which
is called the _boiling point of water_, and is marked 212°. All bodies
that are as hot as boiling water, raise the mercury to 212°; and all
bodies that are as cold as freezing water, make the mercury shrink
to 32°. The space between these two points is divided into 180 equal
parts, each being called a degree. Similar equal divisions are carried
below the point marked 32°, till 0° is reached, and this last point is
called _zero_.
The tube of the thermometer may contain some other fluid than mercury;
but, as this metal is in every respect well fitted for the purpose, it
is most generally used in construction of the instrument.
The thermometer is one of the most useful inventions of science. It
enables travellers to compare the climates of different countries,
and to give us, who stay at home, a distinct idea of them. It is also
necessary in the arts: it enables workmen to apply the exact amount of
heat required in delicate operations, where a little excess might spoil
the whole work.
_Word Exercise._
scald´ing (_skawld-´_)
Mer´cu-ry
in-vĕnt´or
dĕl´i-cate
im-mersed (_im-merst´_)
sen-sā´tions
sur-round´ed
fast´ened (_fas´snd_)
in´stru-ment
con-trac´tion
ex-păn´sion (_eks-păn´shun_)
op-er-a´tions
Fah´ren-heīt
ther-mom´e-ter
tem´per-a-tūre
con-struc´tion