Heat
We experience the sensation of heat when we approach a warm body. The opposite of heat is cold, which merely implies a greater or less deficiency of heat. The two kinds of heat, which are called free, or sensible, and latent, are represented by fire and ice; the free, as in fire, can be felt, while that in ice is latent and cannot be felt. There is heat in all substances, but in those which are called cold it exists in an inferior degree. Some think that heat is not a material substance, but results from the vibrations of the particles of bodies; others believe it to be an exceedingly subtle substance, whose particles repel each other and thus give it a tendency to diffuse itself while they have a strong affinity for other matter. It would appear that heat is closely connected with light, as the one is generally accompanied by the other. That heat has no weight is proved by weighing a piece of ice, and then melting it, the water produced will weigh the same as the ice. The chief sources of heat are the sun, chemical and mechanical action, and electricity. Many speculations have been indulged in as to what composes the sun, that it should continue to give undiminished heat without exhausting the material by which it is supported. That chemical action is a source of heat, may be demonstrated by combining two or more substances to produce a new substance totally different in its nature from either; an increase of temperature alway accompanies such action, as may be proved by mixing sulphuric acid and water in equal quantities; it forms a new substance and gives off heat. Combustion is a chemical union of the oxygen of the atmosphere with the combustible body, or some of its elements. Animal heat is produced by a similar process: when we breathe air is drawn into the lungs, where it comes in contact with the particles of carbon contained in the blood; there is then a chemical union of the carbon with the oxygen of the air inhaled, and, as in the case of combustion, latent heat is evolved. Friction, percussion, and compression are illustrations, showing that mechanical action is a source of heat; and electricity is conclusively shown to be another source of heat, as the heat produced by its action will melt almost any known substance. See COMBUSTION; TEMPERATURE.