ATOMIC THEORY 87 which may therefore be called atomic weights; fourth, that these different atoms have mutual attractions and combine to form chSmical com- pounds, not by interpenetration of their sub- stance, but by atomic juxtaposition. If this idea be admitted, the principles of chemical constancy and definite proportions follow as inevitable consequences. The definite pro- portions in which bodies combine represent the constant ratio between the weights of the combining atoms. The principle of multiple proportions is equally explained, for the suc- cessive additions must be made by whole atoms, and therefore by whole numbers. One atom of carbon unites with one atom of oxy- gen to form carbon monoxide, and with two atoms of oxygen to form carbon dioxide. That the atomic weights of compounds must equal the sum of the atomic weights of their ele- ments follows with equal certainty. Moreover, in the rearrangement of atoms in a body, with- out addition or subtraction of elements, we have a ready explanation of isomeric and allo- tropic changes. The relations of chemical changes to heat, now expressed by the phrase "atomic heat," and their relation to volume, indicated by the phrase "atomic volume," be- come in like manner capable of explanation on the assumptions of the atomic theory. It is a merit and a test of this theory that its re- sources have kept pace with the rapid extension of the science, but it has required to be itself developed for this purpose. In the hands of Dalton it was applied to a few simple funda- mental facts ; it now embraces facts of many orders and of greater complication. At pres- ent the conception of the molecule or the group of combined atoms plays a much more important part than it did at first. Even the atoms of the elements (as will be presently explained) are now conceived not to exist separately, or as units, hut as combined with each other in a molecular condition. An atom is defined as the smallest particle of simple matter that can enter into the composition of a molecule. A molecule is defined as a group of atoms held together by chemical force, and is the smallest particle of any substance that can exist in a free or uncombined state in nature. Molecules are of two kinds : elemental mole- cules, in which the atoms are alike, and com- pound molecules, in which the atoms are un- like. Molecular structure, the outgrowth of the conception of atoms, is now the funda- mental idea by which chemistry and physics are connected. The doctrine of Dalton at first seemed to aft'ord an easy explanation of chemical equivalents, by which one body may replace another, or be substituted for it by simple exchange of atoms. But recent dis- coveries have shown that it fails here and re- i|iiires extension. It was formerly supposed that when one element replaces another in a combination, the substitution always takes place atom for atom, and hence the terms atom and equivalent were regarded as synonymous. But it is now known that this is only true for certain elements, which are accordingly class- ed as monogenic elements. There are others which always take the place of two or more atoms of a monogenic element, and these are termed polygenic elements. This brings us to the new conception of atomicity, which has now become the fundamental idea of the science. To understand it properly, it will be necessary to glance at the steps of chemical theory by which it has been reached. The name of Lavoisier is intimately associated with the first general theory of chemical com- bination. This was the binary or dual system of chemistry. An acid was held to result from the union of a simple body (generally non-me- tallic) with oxygen; an oxide resulted from the combination of oxygen with a metal ; a salt was produced by the union of an acid with an oxide, and this pairing of doubles rep- resents its constitution. In all combinations affinity is assumed to he exerted upon two ele- ments, simple or compound, which attract one another and unite by virtue of opposite proper- ties, all chemical compounds being therefore binary. This is dualism, and the chemical nomenclature was constructed upon the idea. The view proposed by Lavoisier was ably enforced by Berzelius. Electro-chemistry, by which bodies were decomposed into pairs that appeared at opposite poles of the battery, lent powerful aid to the binary theory ; and Berze- lius carried it out by arranging the elements on a scale of antithesis as electro-positive and electro-negative. In 1816 he also devised a new notation, now in general use, by which letters symbolize the elements, and composi- tion can be compendiously represented to the eye by means of formulas. Prof. Wurtz, in his " History of Chemical Theory," says : " By the arrangement of these formulas in which the acid appeared on one side with the train of oxygen atoms belonging to it, and the metallic base on the other with the oxygen united to the metal, Berzelius gave to the dualistic system a degree of precision unknown before his time." But a true scientific theory must embrace all orders of facts to which it is applicable. Dualism was well fortified in mineral chemistry, but it was not easy to bring the complexities of or- ganic chemistry into harmony with it. Berze- lius, however, made this his great task. There were organic acids, organic bases, and organic salts ; and these were represented on the bina- ry plan. Organic radicals were also discovered compounds which played the part of simple elements; and these were subordinated to the binary system. By this theory of compound radicals dualism was extended to organic chemistry, and chemical theory was apparent- ly unified. Yet the victory was far from com- plete. The deeper study of organic compounds led eminent chemists to question the validity of the dual hypothesis as applied to them. A school arose led by Dumas, Laurent, and Ger- hardt, which took a new view of the constitu-
