TRIADS (in chemistry). Until recently, the terms equivalent number and atomic weight were usually regarded by chemists as synonymous. Many recent writers, among whom Laurent (see his Chemical Method, translated by the Cavendish society) stands pre-eminent, have, however, shown that there is an essential difference between them; and this differ ence is fully recognized by prof. Miller, who, in the latest edition of his Chemical Physics, 1863, thus defines it: "The equivalent or combining proportion is an experimental con stant which is independent of theoretical considerations; but the relative atomic weight is necessarily a matter of inference, and may be a number, often a multiple of the equivalent, and selected by the chemist from theoretical considerations, which, being* based partly upon the law of gaseous volumes, partly on chemical grounds, partly on the phenomena of specific heat, seem to require that the atomic weights of a large num ber of the elements, if compared with the atomic weight of hydrogen, should be double of those commonly given.—P. 22. Most chemists of the modern school now agree in arranging the elementary bodies in four groups; namely, 1. Monad or uviequivalent elements (or monads), one atom of which in combination is equivalent to or one atom of hydrogen. In these, the atomic and equivalent numbers are identical. They are twelve in number, and include hydrogen, chlorine, bromine, iodine, silver, etc. 2. Dyad or bieguirale at elements (or dyads), each atom of which, in combining with other bodies, is equivalent to or two atoms of hydrogen. In these, the atomic number is double the equivalent number. This group embraces 25 elements, including oxygen, sulphur, selenium, iron, zinc, etc. 3. or terequivalent elements (or triads), each
atom of which, in combining with other bodies, is equivalent to IL, or three atoms of hydrogen. In this group, which embraces nine elements, including nitrogen, phosphorus, arsenic, etc., the atomic and equivalent numbers are regarded as identical, except in the case of aluminium and rhodium, when the atomic number is doubled. 4. Tetrad or quadrequivalent elements (tetrads), each of which, in combining with other bodies, repre sents H., or four atoms of hydrogen. Their atomic number is double the equivalent number. They are eight in number, including carbon, silicon, tin, etc.
This arrangement of the chemical elements, which is being adopted in all the most recent text-books, has led to the insertion of what are termed dashed symbols, in which the number of dashes which are attached to the symbol for the atom of an element indicates its equivalency or interchangeable value for hydrogen. Thus, Ag is marked with a single dash, to show that silver is a monad, or, in other words, that the atom of silver may be substituted for an atom of hydrogen, so as to combino with an atom of chlorine, the resulting compound being Ag'Cl (chloride of silver): Cu is marked with two, Bi with three, and Si with four dashes, to indicate that they are dyads, triads, and tetrads respectively, or that the atoms of copper, bismuth, and silicon may be substituted for two, three, and four atoms respectively of hydrogen, so as to combine with two, three, and four atoms of chlorine, forming Cu"C12 (chloride of copper), Bi"'Cl3 (chloride of bismuth), and Si'"'Cl., or Sii'Cl. (chloride of silicon). These dashed symbols are rapidly coming into general use.