few saline springs, and in combination with silver; but we have no evidence to show that its uses are important in nature. Hydrogen, again, unites with carbon in various proportions, producing the most dissimilar compounds. The air evolved from stagnant water, and the fire-damp of the coal mine, are both carburetted hydrogen; and the gas which we employ so advantageously for illumination, is the same, holding an additional quantity of carbon in suspension. Naphtha, and a long list of organic bodies, are composed of these two chemical elements. These combinations lead us, naturally, to the consideration of the great chemical phenomena of combustion, which involve, indeed, the influences of all the physical powers. By the application of heat, we produce an intense action in a body said to be combustible; it burns,--a chemical action of the most energetic character is in progress, the elements which constitute the combustible body are decomposed, they unite with some other elementary principles, and new compounds are formed. A body burns--it is entirely dissipated, or it leaves a very small quantity of ashes behind unconsumed, but nothing is lost. Its volatile parts have entered into new arrangements, the form of the body is changed, but its constituents are still playing an important purpose in creation. The ancient notion that fire was an empyreal element, and the Stahlian hypothesis of a phlogistic principle on which all the effects of combustion depended,[219] have both given way to the philosophy of the unfortunate Lavoisier--which has, indeed, been modified in our own times--who showed that combustion is but the development of heat and light under the influence of chemical combination. Combustion was, at one period, thought to be always due to the combination of oxygen with the body burning, but research has shown that vivid combustion may be produced where there is no oxygen. The oxidizable metals burn most energetically in chlorine, and some of them in the vapour of iodine and bromine, and many other unions take place with manifestations of incandescence. Supporters of combustion were, until lately, regarded as bodies distinct from those undergoing combustion. For example, hydrogen was regarded as a combustible body, and oxygen as a supporter of combustion. Such an arrangement is a most illogical one, since we may _burn_ oxygen in an atmosphere of hydrogen, in the same manner as we burn hydrogen in one of oxygen; and so, in all the other cases, the supporter of combustion may be burnt in an atmosphere formed of the, so called, combustible. The ordinary phenomena of combustion are, however, due to the combination of oxygen with the body burning; therefore every instance of oxidization may be regarded as a condition of combustion, the difference being only one of degree.