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.