If the three kinds of air above spoken of be mixed together in a bottle, no change will take place, and if charcoal in fine powder be added to them, still no new substance will be produced. If we take the ash left by a known weight of hay or wheat straw, and mix it with the proper quantities of the four elementary substances, carbon, hydrogen, &c., as shewn in the above table, we shall be unable by this means to form either hay or wheat straw. The elements of which vegetable substances consist, therefore, are not merely _mixed_ together—they are united in some closer and more intimate manner. To this more intimate state of union, the term _chemical combination_ is applied—the elements are said to be _chemically combined_. Thus, when charcoal is burned in the air, it slowly disappears, and forms, as already stated, a kind of air known by the name of carbonic acid gas, which rises into the atmosphere and disappears. Now, this carbonic acid is formed by the _union_ of the carbon (charcoal), while burning, with the oxygen of the atmosphere, and in this new air the two elements, carbon and oxygen, are _chemically combined_. Again, if a piece of wood or a bit of straw, in which the elements are already chemically combined, be burned in the air, these elements are separated and made to assume new states of combination, in which new states they escape into the air and become invisible. When a substance is thus changed by the action of heat, it is said to be _decomposed_, or if it gradually decay and perish by exposure to the air and moisture, it undergoes slow _decomposition_. When, therefore, two or more substances unite together, so as to form a third possessing properties different from both, they enter into chemical union—they form a _chemical combination_ or chemical _compound_. When, on the other hand, one compound body is so changed as to be converted into two or more substances different from itself, it is _decomposed_. Carbon, hydrogen, &c., are chemically combined in the interior of the plant during the formation of wood: wood, again, is decomposed when by the vinegar-maker it is converted among other substances into charcoal and wood-vinegar, and the flour of grain when the brewer or distiller converts it into ardent spirits. CHAPTER II. Form in which these different substances enter into Plants. Properties of the Carbonic, Humic, and Ulmic Acids—of Water, of Ammonia, and of Nitric Acid. Constitution of the Atmosphere. SECTION I.—FORM IN WHICH THE CARBON, ETC. ENTER INTO PLANTS. It is from their food that plants derive the carbon, hydrogen, oxygen, and nitrogen, of which their organic part consists. This food enters partly by the minute pores of their roots, and partly by those which exist in the green part of the leaf and of the young twig. The roots bring up food from the soil, the leaves take it in directly from the air.