HEAT. The fundamental laws of heat, so far as they have yet been discovered, form a very subtle and difficult department of science. Only a few words respecting the action of heat can be admissible in this work.
It is found that expansion, fusion, evapora tion, thermoelectric currents, and various phy siological phenomena, are effects of heat or at least accompany its absorption. Besides the solar rays, heat may be produced artificially by any means which propagate agitations in. ternally in bodies ; hence friction, hammering, percussion, sudden condensation,• chemical combination, and electrical discharges are all proper to produce or rather to develope heat.
The spread of heat throughout a liquid is marked 'by curious effects.' If we place a heated plate on the surface of water in a ves sel, but so as not to touch the edges, a ther mometer placed in the water will indicate little or no alteration of temperature ; and, if the bottom of a vessel of water be heated, the heat will be distributed 'through the liquid only by currents ascending from the heated part. Liquids indeed conduct heat very slowly ; and it is only by the rising of heated particles of water, through their lightness, that' a vessel of water is quickly heated.
The agency of heat in promoting chemical action is important and extensive ; in some cases no combination can take place without it, and in others it greatly facilitates chemical combinations, while- in some instances it de composes compound bodies, and resolves them either into simpler or elementary forms of matter. In the solution of salt in water, an
increase of heat, by increasing the affinity between the solid and the liquid, increases the solvent power. Heat has also great power in modifying as well as in causing chemical action, and different degrees of it produce very opposite effects in some cases. The dilatation of substances by heat is nearly proportional to the increase of temperature, except when they are about to change their physical or chemical states ; thus water near the freezing point expands when the temperature is dimin ished. The following table gives the relative dilatation of different solids from the freezing to the boiling point:— Glass tube .00083 Copper .0017 Crown glass .00089 Brass .0018 Platinum .00093 Silver .0020 Palladium .001 Tin .0022 Cast Iron .0011 Pewter .0023 Steel .0012 Grain tin .0025 Do. tempered .0013 Lead .0028 Gold. .0015 Zinc .0030 Several instruments have been constructed to measure heat ; the most important of which are noticed under PYROMETER and THERMO METER.