One of the most striking and familiar of the laws of heat is what is termed by philosophers “its tendency to an equilibrium.” For instance, if a heated iron ball is suspended nearly in contact with one quite cold, the former in a short time will have imparted so much of its heat to the latter that they will soon become almost of equal temperature. If a penny piece is thrown into a kettle of boiling water it will soon become as hot as the boiling water itself. If a cup of water is exposed to a temperature below 32 degrees, it parts with so much of its natural heat, to come into a state of equilibrium with the medium in which it is placed, that it is converted into ice. These and many more familiar instances might be mentioned as illustrating the law of heat above alluded to. In short, it may be received as one of the best established facts in philosophy, that any substance, no matter what may be its texture or natural qualities, provided it does not possess life, will soon acquire and maintain the same temperature as that of the medium in which it is placed, so long as it continues in that medium. A piece of the metal platinum in the furnace of a glass-house may be kept at a white heat for years; a similar piece of metal, in an ice-house, will remain below 32 degrees so long as it is kept there. It would be unnecessary to notice so particularly these well-known facts, but that they will tend to render more striking the power which living bodies possess of resisting the law to which all unorganized bodies are subject. Any thing possessing life _can maintain a different temperature to the medium in which it lives_. The natural heat of fishes is two or three degrees above that of the water in which they live; the natural heat of creatures which live within the bowels of the earth, like the earth-worm for example, is as much above the usual temperature of the earth; while man himself maintains the heat of his body, as shown by the thermometer placed under the tongue or armpits, at about 98 degrees, under every variety of season, and in every climate under the sun. Were a human being to be kept imprisoned in an ice-house, the heat of his body could never sink to 32 degrees (the freezing point) while life remained. In these mighty reservoirs of ice and cold, the arctic regions, the blood of the rude creatures who exist there is as warm as that of ourselves; and at the torrid zone, where the heat of the sun is almost insupportable, the animal heat of the human frame is only one or two degrees higher than it is at the frozen poles.