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CHEMOTAXIS of chemical compounds are water, composed of hydrogen and oxygen; sugar and alcohol, both of which contain carbon, hydrogen and oxygen; common salt, which is made of sodium and chlorine and has the chemical name of sodium chloride; quartz, called silica or silicon dioxide, which contains silicon and oxygen; and potassium chlorate, which is composed of potassium, chlorine and oxygen. Different samples of any chemical compound, when pure, always contain the same elements in exactly the same proportions by weight or in definite proportions. The same elements often form several compounds by combining in several different proportions; there are hundreds of compounds containing only carbon and hydrogen. It is true, also, that the same elements, combined in exactly the same proportions, may form a number of entirely different substances. This last fact, as well as many other chemical facts, is explained by supposing that the elements are composed of exceedingly small indivisible particles called atoms. All the atoms of each element are believed to be alike, and atoms of different elements are supposed to make up the molecules or smallest possible parts of compounds. The existence of several compounds of the same composition is explained by assuming that the atoms have different arrangements in the molecules. The atomic theory has been developed so elaborately that chemists are able to assign definite relative positions to the atoms in the molecules of a great many compounds, and the relative weights of the atoms of most of the elements have been determined with great accuracy, although their actual weights are unknown. However, since atoms have never been seen (and in all probability never will be seen on account of their small size), their existence cannot be said to be proven.

Most of the substances met with in everyday life are mixtures of chemical compounds. Such are most articles or materials of food and raiment, wood, bricks, paper, glass, rocks, soils, etc. There are a few familiar mixtures of elements, such as metallic alloys, steel and also atmospheric air, which consists chiefly of nitrogen and oxygen.

Our present chemical theories and the greater part of our chemical knowledge have been developed in comparatively recent times. The discovery of oxygen by Priestley in 1774, the correct explanation of combustion shortly afterward by Lavoisier and the founding of the atomic theory by Dalton during the first decade of the last century were important events which mark the beginnings of modern chemical science. Chemical knowledge is still rapidly increasing. In both the inorganic and the organic fields natural substances are being examined, new compounds are being prepared and the laws which govern chemical changes are being studied.

Chemistry has given and is giving much assistance in a practical way to medicine, agriculture, metallurgy and many other branches of art and industry, and still more important advances in these directions are to be expected in the future.  Chem′otax′is (in plants), the sensitiveness of an organism, free to move about, to a one-sided chemical stimulus (see ), to which it responds by taking up a definite attitude with respect to the direction from which the substance is diffusing. Since no plants (except possibly the or slime-moulds, which see) are free to orient themselves thus unless they are immersed in water, it follows that the substance in order to act must be soluble and diffusible in water. Thus the sperms (male cells) of mosses will so place themselves in a diffusing current of sugar particles that, as they swim, they move toward the source of the sugar. Such agencies are believed to determine the movement of the sperms toward the egg in many plants.  Chemot′ropism, the sensitiveness of a plant to a onesided chemical stimulus (see ) to which it responds by changing the rate of its growth in certain regions, and thereby putting the part affected in a new position with respect to the diffusing particles. It differs from chemotaxis only in the nature of the reaction. Diffusing gases or solutes (but usually the latter) may effect the reaction. Thus the growth of the germ-tubes arising from growing spores of fungi is directed by their chemotropism. When, for example, a spore falls upon a leaf on or in which the fungus can develop, it sprouts, and when the young germ-tubes reach the (which see), they turn in and ramify in the interior. Or they may penetrate an epidermal cell at once. It has been shown in both cases that the directive influence is the presence of foods in the leaf. The pollen-tubes are similarly controlled in their growth down the style to the ovules (see ).  Che′ops. See.  Cherbourg. See.  Cherokees, a tribe of North American Indians who were found by the whites in possession of the upper valley of Tennessee River and the rivers and mountains of the Alleghenies, and occupying 64 towns. Meeting the English colonists first, they became their friends and took part with them in the wars against the French. During 1757-61 they were most of the time at war with the English, the trouble growing out of robberies of provisions from the settlers by the Cherokees, who were driven to thievery by hunger on the homeward march, after fighting for the colonists. They yielded to the whites, after losing most of their houses, cattle and horses. During the Revolution they took the side of the British. After the