among metaphysicians, denotes the idea we have of lasting distance, all whose parts exist together.

physiology, the enlargement or increase of bulk in bodies, chiefly by means of heat. This is one of the most general effects of that subtle principle, being common to all bodies whatever, whether solid or fluid. In some few cases, indeed, bodies seem to expand as they grow cold, as water in the act of freezing: but this is found to be owing to the extrication of an infinite 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, or any other solid or fluid substance by means of heat. 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 metals in a certain manner; and is therefore to be accounted a kind of crystallization rather than anything else.

The expansion of bodies by heat is very various, and in solids does not seem to be guided by any certain rule. In the 48th volume of the Philosophical Transactions, Mr Smeaton has given a table of the expansions of many different substances, from which the following particulars are extracted. The degree of heat employed was 180 degrees of Fahrenheit's thermometer, and the expansion is expressed in 10,000th-parts of an English inch.

| Substance | Expansion | |----------------------------------|-----------| | A foot of white glass barometer tube | 100 | | Martial regulus of antimony | 130 | | Blistered steel | 138 | | Hard steel | 147 | | Iron | 151 | | Bismuth | 167 | | Hammered copper | 204 | | A mixture of three parts of copper with one of tin | 218 | | Cast-irons | 225 | | A mixture of 16 parts of irons with one of tin | 229 | | Iron wire | 232 | | Speculum metal | 232 | | Spelter folder, composed of two parts of irons and one of zinc | 247 | | Fine Pewter | 274 | | Grain tin | 298 | | Soft folder, composed of two parts of lead and one of tin | 301 | | A mixture of eight parts of zinc and one of tin, a little hammered | 323 | | Lead | 344 | | Zinc or spelter | 353 | | Zinc hammered an inch per foot | 373 |

From this table it appears, that no rule can be deduced concerning the degree of expansion to which bodies are subject by the same degree of heat, either from their specific gravity or otherwise. Zinc, which is much lighter than lead, expands more with heat; but glass, which is lighter than either, expands much less, while... Exposition while copper, which is heavier than a mixture of brass and tin, expands less.

Of all known substances, those of the aerial 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 it is expanded; though even here we cannot form any general rule. It is certain, however, that the most dense fluids, such as mercury, oil of vitriol, &c., are least expandible than water, spirit of wine, or ether. This last indeed is so easily expanded, that were it not for the pressure of the atmosphere it would be in a continual state of vapour.

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; so that it is 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; neither can we say, 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 show 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. A variety of other curious machines may be constructed on the principle of aerial expansion; of which an account is given under the articles Hydrostatics and Pneumatics.

The expansion of solid bodies is measured by an instrument named the Pyrometer; and the force with which they expand is still greater than that of aerial vapours, the flame of a farthing candle producing 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. On the principle of the expansion of fluids Thermometers are constructed; for an account of which, see that article. For the effects of the different expansions of metals in correcting the errors of machines for measuring time, see the article Pendulum.