EXPANSION. One of the most com mon and obvious effects of heat, which expands or enlarges the bulk of all the forms of matter. The expansion of sol ids byinerease of temperature is compar atively small ; but it may be rendered sensible by carefully measuring the di mensions of any substance when cold, and again when heated : an iron bar, for example, fitted to a gauge, which shows its length and breadth, will no longer pass through the apertures when heated. Among solids the metals are most expans ible and contractile by heat and cold ; but they vary mac in this respect, as shown in the follow' table, which ex hibits the change of imensions which several of them undergo when heated, from the freezing to the boiling point of water :— The average expansion of glass is very nearly the same as that of platinum. The expansibility of different lipids is also very variable : ether, for instance, and alcohol, are more expansible than wa ter, and water more than mercury. The expansibility of mercury is applied to a very useful purpose in the construc tion of the common thermometer. In general all liquids expand and contract in proportion as they are heated and cooled ; but to this law there is a remarkable and anomalous exception with regard to wa ter. When a large thermometer tube is filled with water of the temperature of 60°, and placed in a cold situation, or in a freezing mixture of ice and salt, the water goes on shrinking in the tube, till it has attained the temperature of diout 40°; and then, instead of continuing to contract till it freezes (as is the case with equal liquids), it slowly expands, and ac tually rises in the tube until it congeals. In this case the expansion above 40° and below 40° seems to be equal ; so that the water will be the same bulk at 48° and 32°. This anomalous expansion of water by cold is productive of some important consequences, considered as a natural operation ; for if water, like other fluids, went on increasing in density till it froze, the consequence would be that large bodies of water, instead of being only superficially frozen in winter, would be converted throughout into solid mass es of ice. Let us take a fresh water lake as an example. The earth being in win ter warmer than the air, the heat is with drawn from the surface of the water by the cold breezes that blow over it ; and the whole body of water has its tempera ture lowered to 40°, which is the potibt of its greatest density, and a temperature perfectly congenial to fish and most other aquatic animals. The cold now continues
to operate upon the surface of the water and but, instead of diminishing its bulk, and therefore rendering it headier than the warmer water beneath, it expands it, and renders it lighter ; so that under these circumstances a stratum of ice-cold wa ter (at 32°) will be found lying upon the mass of warmer water beneath it (at 40°). The influence of the cold continuing, the surface of the lake will soon freeze, but the water immediately below the super ficial covering of ice will be found com paratively warm ; and as water is almost a non-conductor of heat, it will be a long time before the ice attains any thickness ; and the whole body of water, if of any depth, can never freeze throughout. In deed, it will be obvious that the retarda tiqn of freezing will be proportional to the depth of water which has to be cooled, and hence some very deep basins or lakes are scarcely ever even covered by ice. As liquids are enlarged and consequent ly rendered specifically lighter by heat, very different effects are produced by ap plying heat to different parts of the ves sels containing them. If the heat be ap plied to the bottom of the vessel, it is soon heated equally throughout, and made to boil; but if the surface only be heated, it may then be boiled and evapo rated, while the lower parts remain quite cold.
Aetiform bodies and vapors are the most expansible forms of matter, and they present an important peculiarity ; for in other substances each individual has its own degree of expansion and con traction, whereas all pure aeriform bodies expand and contract alike ; so that if we accurately determine the expansion and contraction of any one of them, that knowledge applies to all the rest. 100 measures of air, when heated from the freezing to the boiling point of water, suffer an increase of bulk equal to 37•a parts ; so that 100 cubic feet of air at 32° become dilated to 1371 cubic feet at 212°.