88 ATOMIC THEORY tion of organic bodies. Its first idea was the doctrine of substitutions, and in its application a breacli was made at the outset in the electro- chemical theory. It was found that chlorine, a powerful electro-negative element, could re- place hydrogen, a strong electro-positive ele- ment, in an organic compound, playing the same part and not altering the character of the compound. The new view, rejecting dual- ism, regarded organic bodies as units, or as unitary structures ; and their changes by sub- stitution were likened to the alteration of an edifice by successively removing its individual bricks and stones and replacing them by others. Laurent compared organic compounds to crystals, whose angles and edges may be replaced by new atoms or groups of atoms, while the typical form is preserved. Thus to the dualistic point of view was opposed the unitary system ; to the idea of combination resulting from addition of elements was op- posed that of compounds formed by substitu- tion of elements. An acid is changed to a salt by substituting a metal for its hydrogen, with- out destroying its molecular structure. A salt is no longer to be regarded as a binary com- pound, containing an acid on the one side and an oxide on the other ; it is a whole, a single group of atoms, among which are one or more atoms of metal capable of being exchanged for other metallic atoms or for hydrogen. This view led to the theory of chemical types, in which certain substances are taken as patterns of molecular structure with which analogous bodies are classified. Thus we have the water type, the hydrogen type, and the ammonia type, under which bodies are grouped with no reference to their former relationships. The binary theory here disappears, and substances are brought together not so much on the prin- ciple of composition or atomic arrangement, as by analogies of reaction and decomposition. But the doctrine of types was transitional, and soon developed into the completer theory of atomicity, by which is meant combining capa- city. For example, there are some acids which require for saturation only one equivalent of a certain base; there are others which require two equivalents of the same base to saturate them ; and others still which demand three. Now these acids are clearly not equivalents of each other, their capacities of combination va- rying as 1, 2, 3 ; and they are therefore said to have different atomicities. This conception of [ the varying combining powers of bodies, as a controlling chemical principle, was worked out in the field of organic chemistry ; but it is now extended to the inorganic elements, and offers a new system of classification and a new chem- ical method. In the new chemistry the ele- ments are arranged into six groups, although some add a seventh. These are named mo- j nads, dyads, triads, tetrads, pentads, and hex- ! ads terms expressive of their several combin- ; ing capacities. Monads, of which hydrogen, chlorine, and potassium are examples, are monogenic, that is, they can combine only with single atoms. All the rest are polygenic, that is, they can combine with 2, 3, 4, 5, or 6 monogenic elements or their equivalents. Mole- cules are also designated as monatomic, di- atomic, triatomic, tetratomic, pentatomic, and hexatomic. For equivalence, which represent- ed the old idea, the term valence is coming into use; and a series of words is derived from it describing the groups as univalent, bivalent, trivalent, quadrivalent, quinquivalent, and sexi- valent, while the atomicities above univalence are termed multivalent. The varying equiva- lence, valence, or combining power of atoms is represented in several ways by which the idea is made clear. The graphic symbol of an atom is a circle with lines radiating from it, called bonds, which indicate the valence or atomicity. They are represented as follows, the first line giving their names, the second their symbols, and the third examples : Monad. Dyad. Triad. Tetrad. Pentad. Hexaci. Hydrogen. Oxygen. Boron. Carbon. Nitrogen. Sulphur. Water, OHj, would be thus represented by graphic formula: Hydrogen has as it were but a single pole of attraction, represented by a single bond, while oxygen has two poles and two bonds. The attractions of the two atoms of monatomic hydrogen are satisfied by the two attractions of diatomic oxygen. So carbon-dioxide, CO S , may be Here the represented thus : four attractions of tetratomic carbon are satu- rated by those of the two atoms of diatomic oxygen. Marsh gas, CH<, is thus represented : The circle may be omitted, H ) and the bonds connected di- rectly with the letters, thus, II, -O-, C-, it being immaterial how the bonds are arranged. The compo- sition of water will then be represented thus, H O H, and carbon-dioxide O = C=O. The atomicity is often represented as follows by dashes : H', O", B'", C"", N'"", 8""" ; or again thus by Roman numerals : HP, 0", B m , O lv , N T , S VI . In chemical changes and the formation of new compounds all attractions require to be satisfied every bond engaged. This fact fixes a limit to combination, for cer- tain groupings become impossible. One atom of a monad cannot unite with one atom of a dyad, because one attraction cannot neutralize two. It takes two atoms of a monad to form a compound with an atom of a dyad ; four atoms of a monad or two atoms of a dyad are required to saturate a tetrad ; but in each case all the polarities have to be provided for.
