(347.) The most obvious sources of heat are, the sun, fire, animal life, fermentations, violent chemical actions of all kinds, friction, percussion, lightning, or the electric discharge, in whatever manner produced, the sudden condensation of air, and others, so numerous, and so varied, as to show the extensive and important part it has to perform in the economy of nature. The discoveries of chemists, however, have referred most of these to the general head of chemical combination. Thus, fire, or the combustion of inflammable bodies, is nothing more than a violent chemical action attending the combination of their ingredients with the oxygen of the air. Animal heat is, in like manner, referable to a process bearing no remote analogy to a slow combustion, by which a portion of carbon, an inflammable principle existing in the blood, is united with the oxygen of the air in respiration; and thus carried off from the system: fermentation is nothing more than a decomposition of chemical elements loosely united, and their re-union in a more permanent state of combination. The analogy between the sun and terrestrial fire is so natural as to have been chosen by Newton to exemplify the irresistible force of an inference derived from that principle. But the nature of the sun and the mode in which its wonderful supply of light and heat is maintained are involved in a mystery which every discovery that has been made either in chemistry or optics, so far from elucidating, seems only to render more profound. Friction as a source of heat is well known: we rub our hands to warm them, and we grease the axles of carriage-wheels to prevent their setting fire to the wood; an accident which, in spite of this precaution, does sometimes happen. But the effect of friction, as a means of producing heat with little or no consumption of materials, was not fully understood till made the subject of direct experiment by count Rumford, whose results appear to have established the extraordinary fact, that an unlimited supply of heat may be derived by friction from the same materials. Condensation, whether of air by pressure, or of metals by percussion, is another powerful source of heat. Thus, iron may be so dexterously hammered as to become red-hot, and the rapid condensation of a confined portion of air will set tinder on fire. (348.) The most violent heats known are produced by the concentration of the solar rays by burning glasses,--by the combustion of oxygen and hydrogen gases mixed in the exact proportion in which they combine to produce water,--and by the discharge of a continued and copious current of electricity through a small conductor. As these three sources of heat are independent of each other, and each capable of being brought into action in a very confined space, there seems no reason why they might not all three be applied at once at the same point, by which means, probably, effects would be produced infinitely surpassing any hitherto witnessed.