EXPANSION

, is the dilating, stretching, or spreading out of a body; whether from any external cause, as the cause of rarefaction, or from an internal cause, as elasticity. Bodies naturally expand by heat beyond their dimensions when cold; and hence it happens that their dimensions and specific gravities are different in different temperatures and seasons of the year. Air compressed or condensed, as soon as the compressing or condensing force is removed, expands itself by its elastic power to its former dimensions.

In some few cases indeed bodies seem to expand as they grow cold, as water in the act of freezing: but it seems this is owing to the extrication of a number of air bubbles from the fluid at a certain time; and is not at all a regular and gradual expansion like that of metals, &c, by means of heat. Mr. Boyle, in his History of Cold, says that ice takes up one 12th part more space than water; but by Major Williams's experiments on the force of freezing water, I have found it occupies but about the 17th or 18th part more space. Transac. of the R. Soc. of Edinb. vol. 2, pa. 28. In certain metals also, an Expansion takes place when they pass from a fluid to a solid state: but this too is not to be accounted any proper effect of cold, but of the arrangement of the parts of the metal in a certain manner; and is therefore to be accounted a kind of crystallization, rather than any thing else.

The Expansion of different bodies by heat is very various; and many experiments upon it are to be met with in the volumes of the Philos. Trans. and elsewhere. In the 48th vol. in particular, Mr. Smeaton has given a table of the Expansion of many different substances, as determined by experiment, from which the following particulars are extracted. Where it is to be noted, that the quantities of Expansion which answer to 180 degrees of Fahrenheit's thermometer, are expressed in ten-thousandth parts of an English inch, each substance being 1 foot or 12 inches in length.

By other experiments too it has been found that, for each degree of heat of the thermometer, mercury, wa- ter, and air, expand by the following parts of their own bulk, viz,

From the foregoing table it appears, that there is no general rule for the degree of Expansion to which bodies are subject by the same degree of heat, either from their specific gravity or otherwise. Zink, which is much lighter than lead, expands more with heat; while glass, which is lighter than either, expands much less; and copper, which is heavier than a mixture of brass and tin, expands less.

It seems too that metals observe a proportion of Expansion in a fluid state, quite different from what they do in a solid one: For regulus of antimony seemed to shrink in fixing, after being melted, considerably more than zink.

But of all known substances, those of the aërial kind expand most by an equal degree of heat; and in general the greater quantity of latent heat that any substance contains, the more easily is it expanded; though even here no general rule can be formed. It is indeed certain that the densest fluids, such as mercury, oil of vitriol, &c, are less expansible than water, spirit of wine, or ether. Which last is so easily expanded, that were it not for the pressure of the atmosphere it would be in a continual state of vapour. And indeed this is the case, in some measure, with perhaps all fluids; as it has been found, by experiments with the best air-pumps, that water, and other fluids, ascend in vapours the more as the exhaustion is the more perfect; from which it would seem that water would wholly rise in vapour, in any temperature, if the pressure of the atmosphere was entirely taken off.

After bodies are reduced to a vaporous state, their Expansion seems to go on without any limitation, in proportion to the degree of heat applied; though it may be impossible to say what would be the ultimate effects of that principle upon them in this way. The force with which these vapours expand on the application of high degrees, is very great; nor does it appear that any obstacle whatever is insuperable by them.

On this principle depend the steam engines, so much used in various mechanical operations; likewise some hydraulic machines; and the instruments called manometers, which shew the variation of gravity in the external atmosphere, by the expansion or condensation of a small quantity of air confined in a proper vessel. On this principle also, perpetual movements might be constructed similar to those invented by Mr. Coxe, on the principle of the barometer. And a variety of other curious machines may be constructed on the principle of aërial expansion; an account of some of which is given under Hydrostatics and Pneumatics.

On the principle of the Expansion of sluids are constructed Thermometers. And for the effects of the different Expansions of metals in correcting the errors of machines for measuring time, see the article PENDULUM.

The Expansion of solid bodies is measured by an instrument called the Pyrometer; and the force with which they expand is still greater than that of aërial vapours; the flame of a farthing candle produces an | Expansion in a bar of iron capable of counteracting a weight of 500 pounds. The quantity of expansion however is so small, that it has never been applied to the movement of any mechanical engine.