18. _Caloric_.—Every one is acquainted with the different sensations of heat and cold. That matter which produces on our bodies the sensation of _heat_ has the name of _Caloric_; heat being only an effect, of which caloric is the cause. This is extended in a greater or less degree through the whole extent of space, and penetrates into the interior of even the most solid bodies: in so doing it expands the particles of which they are composed, augments their bulk, and diminishes their solidity. The sun is the principal fountain from which the earth is supplied with this fluid; and it passes thence to us at the rate of 12,000,000 of miles per minute. The defect of caloric in any substance occasions the sensation called cold. Were the world deprived of caloric, every species of organized being would, from that moment, cease to exist. It is the cause of all fluidity: to it every production of the earth has been most essentially indebted, even for its form and structure; and in no respect do the power and goodness of the Almighty appear more conspicuous than in the creation, dispersion, and continuance, of this most subtile and astonishing fluid. 19. All the various substances with which we are acquainted must be considered either as solid or fluid. Every substance is defined to be a _solid_ in which the parts are so united or connected that it requires an external force to separate them. A _fluid_, on the contrary, is a body the parts of which are so loosely connected that they not only yield easily to any force impressed upon them, but also move freely amongst each other; and every fluid is a combination of caloric with some other substance. 20. Fluids are of two kinds: one of these, called _liquids_, have, when at rest, a smooth and distinct surface, and are distinguishable both by the sight and touch; the other, denominated _gas_, or _gaseous fluids_, have the appearance of air, and are not perceptible either to the sight or touch, except under certain circumstances. The latter are principally oxygen (21), azote or nitrogen, and hydrogen (45). We shall at present have occasion to speak only of the first. GASEOUS FLUIDS. 21. _Oxygen_, like caloric, is a fluid never found in an uncombined state. It forms one of the component parts of the air that we breathe, and of the water we drink; but it approaches nearest to a state of purity in combination with caloric (18), when it has the name of _oxygen gas_. It was formerly called _vital air_, because no breathing animal can live for a moment in any air or gas which has not in it a mixture of oxygen; every kind of combustible burns with great splendour in it, and without it ceases to burn. It unites with a great number of substances, and changes both their appearance and properties in a very remarkable manner. Of the metals it entirely destroys the metallic lustre, and gives them an earthy form and texture. Substances in this state have the name of _oxides_.