When made red hot, it will be found so much enlarged as to be incapable of passing through the hole, or fitting into the gauge, thus proving that it has been enlarged in all its dimensions. The cubic or solid expansion is always three times greater than the linear. For example, if the expansion of a rod of steel be one-tenth of its length, the whole expansion will be three-tenths of its bulk. The expansion of liquids may be made evident by filling a Florence flask, or any bottle with a narrow neck, with cold water, and then applying heat to it; the fluid will soon be observed to flow over the mouth of the bottle in consequence of its increased bulk. This fact is also familiarly illustrated by means of an ordinary tea-kettle, which, if it be quite filled with cold water, will be unable to contain the increased bulk occasioned by heat, and will consequently discharge a portion of its contents over the hearth long before the water reaches its boiling point. The expansion of air may be shown by a similar apparatus. If the flask full of air be inverted with its mouth under water, and the flame of a lamp or candle be applied to it, the air will be expanded, and will be seen escaping from the mouth of the flask in large bubbles.

If the flame be removed, the air will cool, gradually contract its dimensions, and the water will rush up into the vessel to fill the space vacated by the air. The expansion of solid bodies is ascertained by what is called a pyrometer, for the construction of which see its name. The most general facts connected with expansion are as follows: - 1. Nearly all solids, liquids, and gases, are expanded by heat, and contracted by cold; and of these, the gases are most expansible, and solid bodies least. 2. Different solids and liquids are differently expanded by the same degree of heat, but gases and vapours are equally and equably expanded by equal portions of heat. 3. In the same body, whether solid or liquid, the expansion by a given quantity of heat, is greater at higher than at lower temperatures; but in gases and vapours, the expansions are equal, by equal additions of heat at all temperatures. 4. The expansion of atmospheric air, gases, and vapours, not in contact with the liquids from which they have been generated, is equal to one four hundred and eightieth part of the bulk they occupy at 32° of Fahrenheit's thermometer.

The absolute dilatation in length of several of the most generally used substances, is shown in the following table • -

Expansion of liquids in bulk.

One of the most important applications of the expansibility of liquids, is in the construction of the thermometer, which consists essentially of a closed tube, containing a liquid, the expansion of which can be conveniently observed. To construct a thermometer, a glass tube of very small bore is usually taken, and a bulb, globular, cylindrical, or conical, is blown at one end, which bulb and part of the tube are then filled with perfectly pure mercury. The tube must then be hermetically sealed by melting the top by means of a blow-pipe flame. Previous to closing the top, however, the mercury must be exposed to heat so as to completely fill the tube; and while the tube is full, the sealing must be effected. When the mercury cools, the surface will subside, and a vacuum will be left in the upper part of the tube. The thermometer is then complete, except its graduation; this is accomplished as follows: - The bulb is to be plunged into snow or ice that is just melting, and a mark made on the tube at the point at which the mercury stands; this is called the freezing point; and in the thermometers common in this country, called Fahrenheit's (from the inventor's name), it is marked 32°.

The tube must next be placed in boiling water, or its steam, in such a way that the whole of the mercury may be exposed to heat. When the mercury has risen and is stationary, another mark must be made; this denotes the boiling point of water, and is marked 212°. The space between the boiling and freezing points must next be divided into 180 equal parts, and the graduation is complete. In the centigrade thermometer used in France, the freezing point is marked 0°, and the boiling point 100°, the intervals being divided into 100 equal parts or degrees. One degree of Fahrenheit is therefore equal to four-ninths of the centigrade; and one of the centigrade to nine-fourths, or 21/4° Fahrenheit. This is but a brief and general view. For further information, see Thermometer.

It has been already stated that fluidity is the result of heat. Every solid may be liquefied; and many of them, as well as liquids, may be vaporized at a certain elevation of temperature; and there can be little doubt that every known liquid may be solidified if we possessed the means of sufficiently reducing the temperature. Several of the gases, by the united operation of cold and pressure, have already been reduced to the fluid state, and nearly every known liquid may be frozen. The temperature at which a solid body assumes the liquid state is palled its fusing or melting point. If the substance be ordinarily in the liquid state, this point will be its freezing, concreting, or congealing point. The concreting or congealing temperatures of various substances will be found in the following table: -

The temperature at which bodies melt is fixed, but the congealing point is modified by circumstances. Many liquids may be cooled considerably below their usual freezing points, before they assume the solid state. This is the case with water, which may be cooled 10°, or even 20° below its congealing point without freezing; but if a small spicula of ice be dropped into it, or the water caused to vibrate, it will instantly solidify. The same phenomena occur with saline solutions. A hot saturated solution of sulphate of soda may be cooled to 50" under a film of oil, and it will remain liquid, and bear to be moved about in the hand without any change; but if the vessel containing it be placed on a vibrating table, crystallization will instantly take place. One remarkable fact attends the cooling of bodies below their usual freezing point; viz. at the instant solidification occurs, the temperature of the mass rises to the proper freezing point. Thus, if a portion of water be placed in an atmosphere of 21°, the liquid will cool and remain fluid at this temperature, till, by touching it with a piece of ice, or causing it to vibrate, we make it freeze, when it instantly rises to 32°, or 11° above the surrounding medium.