Friction is, therefore, one means of producing distributable heat, which is also exemplified very frequently in the axis of a carriage wheel; of mill work; in the rubbing of wood, when turned on its axis in a lathe, by which turners ornament their work with black rings; rubbing a cord very swiftly backwards and forwards against a post or tree, or letting it run over a boat, &c. as in the whale fishery; the motion of two iron plates against each other, pressing them at the same time, &c. The great object in the construction of machines is to avoid, or lessen the degree of friction. See Hatchette, Vince, and Gregory. Count Rumford (Nicholson's Journal, 4th edit. ii, 106), and professor Pictet (_Essai sur le Feu_, chap. ix.) have made some valuable experiments on heat evolved by friction. The sun is one great source of caloric. In whatever mode it produces it, whether by giving it out from its own substance, by the action of light on the air that surrounds the globe; by the concentration of calorific rays by means of the atmosphere, acting as a lens; or by putting caloric in the distributable state, always pre-existing in some other, as in a state of rest, are questions, which, in our present state of knowledge, we are unable to solve. We know the fact, and that the caloric is of the same nature as that obtained by combustion.[3] Combustion is a process by which caloric is put in a distributable state. The opinion of Stahl and others, that all combustible bodies contained a certain principle called phlogiston, to which they owed their combustibility, and that combustion was nothing more than a separation of this principle, gave rise to the phlogistic or Stahlian theory, which was afterwards modified by Dr. Priestley. But his theory is equally untenable. Kirwan's opinion was no less vague, although he substituted hydrogen for phlogiston. The Lavoiserian, or antiphlogistic theory overturned the Stahlian. According to this theory, a combustible in burning unites with oxygen, and heat and light are given out by the gas, and not from the combustible. According to a modified theory of the above, by Dr. Thomson, caloric is evolved by the gas, and light from the burning body. Without noticing the instances, in which this theory, as a general one, is insufficient to explain the cause of combustion, or of the production of heat and light, we will merely remark, that bodies which support combustion are called supporters, as oxygen gas, chlorine gas, &c. and those, that undergo this change, are named combustible bodies.