Until recently it has been supposed that the chemist could destroy organic substances, but that he could not create them. This idea is no longer held. A great number of the compounds formed by plants and animals have been produced in the chemist’s laboratory, without the aid of vital force. While it is undoubtedly true that many of the compounds found in plants and animals are not _necessarily_ related to organisms, there are usually some plain facts which differentiate organic compounds from inorganic. Among these may be named the following: Organic substances are usually composed of but few elements; oxygen, nitrogen, hydrogen and carbon constituting almost all their material. Ten other elements are very sparingly distributed in them. _Sixty-six_ elements enter into the formation of inorganic matter. The _atomic_ structure of the former is very complex; that of the latter is simple. For example: A molecule of the white of egg contains 222 atoms, while a molecule of salt has but two. Again, the compounds of organic existences are innumerable. Inorganic compounds are _comparatively_ few. The former are unstable, on account of the presence of nitrogen, while the latter are fixed and quite permanent. The former are also distinguished for the many examples of isomerism they furnish. Isomeric compounds are those formed of the same elements in the same proportions. Thus, camphene, the oils of bergamot, juniper, birch, black pepper, lemon, cloves, turpentine, ginger, cubebs, orange, and many others are isomeric, each one being composed of ten atoms of carbon and sixteen of hydrogen. The difference in these volatile oils is supposed to be due to a variation of the _arrangement_ of the atoms composing them. Let us now briefly consider the FOOD OF PLANTS. This is obtained from the air and earth. The former supplies carbonic acid, and water in the form of vapor, through the stomata of the leaves; these are little mouths or breathing pores, chiefly situated on the under side of the leaf. They vary in number from one thousand to one hundred and seventy thousand to the square inch. An apple-tree leaf of average size has one hundred thousand pores. The old elm at Cambridge, under which Washington stood while reviewing the Continental army, has been estimated to produce a crop of seven million leaves, thus exposing a surface of five acres, and therefore furnishing billions of stomata. If the amount of carbonic acid gas in the air were much increased, all higher forms of animal life would perish. If it were materially lessened, vegetation would soon wither and die, involving the death of all animals, from lack of food. Plants derive the element carbon from this gas. [Illustration: A PLANT STARTING IN LIFE.] According to Chevandier, an acre of beech forest annually absorbs three and one half tons of carbonic acid gas, and from this eliminates about one ton of carbon.