The simplest case which we can consider is that of a compound consisting of two monovalent atoms, such as hydrogen chloride. Here we have the structural formula, H Cl. A compound of a dyad with two monad atoms, such as water, or its analogue, hydrogen sulphide, must have the formula, H O H, or H S H. The compound of a triad with three monad atoms, as, for example, ammonia, would be written
/H H N/ ; and of a tetrad with four monads,
where an atom of carbon is the tetrad, and the compound is named methane, or "marsh-gas." The atom of sulphur is, however, not always divalent ; it is sometimes tetra valent, as in its compounds with chlorine and with oxygen. In the first case, tetra-chloride of sulphur has the formula,
78^ ; and in the latter, sulphur dioxide is repre CK \C1
sented by the formula O = S = O. Sulphur dioxide unites directly with chlorine on exposure of a mixture of the two gases to sunlight, forming a compound named sulphuryl chloride, which has the empirical formula, SO 9 C1 ; in this compound sulphur is regarded as a hexad, hence the
structural formula must be /$C * Now, sulphuryl
chloride reacts at once with water when they are brought into contact, and sulphuric acid is produced along with hydrogen chloride. This change can be represented structurally by the equation :
H O H Ck ,O H-C1 H Ov ,O
H O H Cl/ ^O H-C1 H O/ V)'
The chlorine atoms of the sulphuryl chloride have com bined with two of the hydrogen atoms of two molecules of water, leaving the residues O H, which are termed " hydroxyl groups ; " these have taken the place of the chlorine atoms, forming sulphuryl hydroxide, or, as it is commonly termed, sulphuric acid. If the foregoing repre sentation is correct, then an intermediate substance should exist, which may be named " sulphuryl hydroxy-chloride," and which should contain a chlorine atom and a hydroxyl group, each in union with sulphuryl. Such a body has been prepared by the direct union of sulphur trioxide, where sulphur is in combination with three atoms of oxygen, with hydrogen chloride. But here there must be a transposition of the hydrogen atom, as is evident from the equation
H ,,0 H-0 V ^O
In a similar manner to the above schemes, the relations of the atoms in compounds may be traced out, but sometimes it is difficult to decide regarding the structure. Here is an instance. The specific heat of the element barium shows that it possesses an atomic weight not far removed from 137 ; the analysis of its chloride leads to the fact that 137/2 grams of barium are in combination with 35.5 grams of chlorine, and 35.5 is known to be the equivalent of chlorine; hence 63.5 is the equivalent of barium, and
63. 5x2 = 137 is its atomic weight. Ordinary oxide of barium corresponds with this, for it contains 137 grams of barium in combination with 16 grams of oxygen ; hence we accept barium as a dyad. But if barium oxide be heated to dull redness in a current of oxygen, another atom of oxygen combines with the oxide, and in the compound BaO , 137 grams of barium are combined with 32 grams of oxygen. Is barium a tetrad ?
Among all the numerous compounds of barium, no one is known in which one atom of barium is combined with more than two atoms of a monad ; when barium dioxide is treated with hydrochloric acid, for example, two atoms of oxygen are not replaced by four atoms of chlorine, but the change is BaO 2 + 4 HC1 = BaCl 2 4H 2 O 2 .
Hydrogen dioxide is produced. Now the formula of hydrogen dioxide has been proved by the freezing-point method to be H 2 O 2 , and not HO ; hence it may be sup posed that it consists of two hydroxyl groups in union with each other, thus : H O O H ; in this case, barium dioxide would be BaO2 , the two atoms of oxygen being themselves united together ; and there are many instances of similar union. But it may also be held that one of the atoms of oxygen is a tetrad, the other remaining a
H \ dyad, thus : ;>O=O ; whence barium dioxide would be
Ba=O = O. Both of these views can be supported by arguments, and it is an open question which has a claim to preference. It is certain, however, that barium is not a tetrad.
In other instances, it must be confessed that the evidence is by no means so clear, and there is then considerable doubt as regards the correct classification of the elements concerned.
It must not be forgotten that we have as yet no clear conception as to the cause of valency ; at present we accept the facts, and endeavour to use them as a guide to the classification of compounds.
Were all the elements to be capable of combining with each other, it is easily seen that the number of compounds would be prodigious, and that no mind could possibly hope to grapple with them ; but it happens that only a certain number of elements forms well-defined compounds with the rest, and the grouping of compounds is thus not so difficult a task as might be supposed. The classes are the following :