The flame exhibited in the burning of charcoal and phosphorus is merely the ignition of the solid particles of these bodies. At a certain elevation of temperature, about 800° of Fahr., all solid bodies begin to give out light, and the same effect is produced in vacuo by transmitting voltaic electricity through a metallic wire. The phosphorescence of minerals is another source of light. If fluor spar be coarsely pounded and placed upon a mass of iron heated below redness, it will give out a beautiful green light; this property is possessed by quartz, topaz, phosphate of lime, and a variety of other minerals. There is also a clas3 of bodies called solar phosphoric, which emit light upon exposure to any highly luminous body; the most powerful of these is a compound discovered by Canton. If three parts of calcined oyster shells in powder are mixed with one of flour of sulphur, and the mixture rammed into a crucible, and ignited for half an hour, we shall find that the bright parts, on exposure to the sun-beam, or to the common daylight, or to an electric explosion, will acquire the faculty of shining in the dark so as to render visible the figures on the dial-plate of a watch.

After a while they will cease to shine; but if the powder be kept in a well corked phial, a new exposure to the sun-beam will restore the luminescence. When an electric discharge is transmitted along the surfaces of certain bodies, a somewhat durable phosphorescence is occasioned; thus - Canton's pyrophorous produces more light by this treatment than any other body, but nearly every native mineral, except metallic ores and metals, become luminous by an electric explosion. Light is also emitted in certain chemical changes, where no sensible heat is perceived. Marine animals, both living and dead, emit light; as the shell fish called pholas, the medusa phosphorea, and other molluscae. When deprived of life, marine fishes in general seem to abound in this kind of light; among insects, also, several species of fulgora, or lantern-fly, and of lampyris, or glow-worm. Rotten wood and peat earth also emit light copiously. This luminous matter may be separated from the bodies of animals by immersing them for a short time in dilute saline solutions; one part of sulphate of magnesia in eight of water is the most convenient menstruum for this purpose.

Fresh water, spirit of wine, and dilute acids, destroy the luminous property altogether, while a gentle heat, and oxygen gas, increase the brightness of the phosphorescence.

The next important agent in chemical investigations is heat. Many chemists have considered heat or caloric as a material substance, capable of entering into combination with all other kinds of matter; but recent investigations seem to put it and light upon the same ground, and show them to be but certain states of matter, brought about by the undulations of an exceedingly subtile ether, which is considered to fill all the space in the universe not occupied by other matter. Heat appears to be an antagonist principle to attraction; while the latter binds together, the former tends to separate, the parts of bodies, first expanding them, then reducing them to the liquid state, and finally to that of gas or vapour. Sometimes it decomposes a body, separating it into its proximate or ultimate elements: so powerful, indeed, is heat as a repulsive agent, that there can be no doubt that, if the temperature of the globe were sufficiently increased, every solid mass on its surface would become liquid, and every liquid assume the state of vapour.

We have an illustration of this consequence in the different states of the same body on different parts of the earth's surface.

Thus some bodies, as butter, oil, water, etc. which exist only as solids at the poles of the earth, are perfectly liquid at the equator. Ether, which is a liquid in this latitude, can only exist as a vapour in the torrid regions of the earth. Mercury also, which we see always in the liquid state, may, by exposure to the intense cold of a northern sky, be converted into a solid metal like lead. The most immediate and general effect of heat is expansion. If a poker or an iron ball be placed in a fire, it becomes heated, and assumes that state which is called ignition; at which time its temperature is nearly equal to that of the surrounding fuel. In like manner, if a cup of cold water be placed within a vessel of hot water, the two speedily become of the same temperature, the one giving out caloric, and the other receiving it. If now either of these bodies that have changed their temperature be examined, it will be found to have changed its bulk also. This expansion of bodies by heat, and their consequent contraction by cold, are facts of the highest importance in the arts.

The amount of expansion in different substances by a known increase of temperature, has therefore been carefully studied, and a variety of results, more or less accurate, have been obtained by different experimentalists. The property of expansion by heat, and contraction by cold, appears to belong to nearly all bodies upon which experiments have been made; at least in that class of bodies which compose the mineral kingdom, for among vegetables the effects of heat are so mixed with other effects, that it is difficult to distinguish them. Some minerals, too, as clay, and several metallic substances, have been considered as exceptions to the general law; but a more minute investigation has shown that they are only apparent. Clay contracts in consequence of the expulsion of moisture, and the incipient fusion that occurs; and some of the metals, as iron, bismuth, antimony, in fusion, expand in the act of setting or solidifying, but afterwards contract as they cool. The force with which bodies expand is so great as to overcome every resistance that is opposed to it. The general fact of the expansion of solid bodies by heat may be seen by taking a metal cylinder a b that will just fit into a gauge c d, and pass through a circular hole e when cold.