This series includes the oxides (comprising under this term acids, bases, and salts), sulphides, selenides, and tellurides of all the monad radicles, such as the alcohols, the hydrosulphuric ethers and mercaptans, the simple and compound ethers, the double ethers, the monobasic organic acids and their anhydrides: for instance,

H'

includes the nitrides, phosphides,

as well as arsenides and anti

3. The ammonia type H' N"", monides of the alcohol radicles,

The members of this group have hitherto been but little studied; they comprise compounds like carbonic anhydride and some of the more complex organo-metallic bodies..

It will be seen, as we advance, that bodies are also known which are formed upon condensed types, or upon the plan of the preceding types doubled, tripled, quadrupled, &c., such as the type of 2 molecules of water

H2

→ condensed into one, as is more 2 на

fully explained at p. 59; among these, for instance, are

whilst there are others formed upon the type of 3 molecules of

56

CHEMICAL TYPES.

Glycerin

and so on.

(E,H.)"
H3

These condensed types are represented only in the case of compounds which contain polybasic radicles, such as sulphuric (SO)" and glycyl, (,H.)"" as in the instances just cited, where the radicle serves as the link to bind the several molecules

into one.

Each of the four substances, hydrogen, water, ammonia, and marsh gas, thus selected as types to one of which the various chemical compounds are referred, it will be seen is a compound of hydrogen; and other bodies are supposed to be formed from these typical substances by the substitution of a portion, or of the whole of the hydrogen of the type, by hydrocarbons or by other simple or compound radicles. For the sake of precision it is often found to be necessary to distinguish the unaltered hydrogen of the type from any hydrogen which may have been subsequently introduced into the molecule.

5

Alcohol, for example, is said to be a compound formed upon the water type by the introduction of a hydrocarbon ЄH, in the place of one of the original atoms of hydrogen in the type; alcohol being

H

H
→; whilst water is Ꮎ .

H

In such cases the original hydrogen in the type is distinguished as typical hydrogen.

Many compounds, not included under the preceding types, may be referred to the union of two of the foregoing types with each other, as was first suggested by Williamson, such compounds being represented as derived from mixed types. Kekulé, for example (Lehrbuch der Org. Chemie, i. p. 121,) proposes to consider the amidated acids, such as the carbamic and the oxamic, as formed upon the combination of the types of ammonia and

the tie between the two typical groups is in one dibasic radicle (CO)", in the other (,)".

case the

(1064) Theory of Polybasic Elements.-The tendency of bodies

THEORY OF POLYAD OR POLYBASIC ELEMENTS.

57

to assume these typical forms of combination is supposed to be due to the existence of special peculiarities in the nature of the elementary bodies themselves.

This principle, first suggested by the consideration of certain organic compounds, has since been extensively applied to the explanation of the formation of chemical compounds in general. A new theory of chemical combination has been founded upon it; and although the theory has not yet been worked out in all its details, it has, in the hands of Kekulé, Frankland, Wurtz, and many other eminent chemists, already proved a fertile source of new discoveries, and has led to generalizations of great interest and growing importance.

Although, therefore, it is still of a speculative character, it will be necessary to give a sketch of the general features of the view of the atomicity or equivalency of elements which is at present under discussion amongst chemists.

Observation has shown that each elementary substance has a tendency to unite with the other elements in a particular manner, and the special peculiarities of these combinations have rendered it necessary to arrange the more important of the elements in six principal groups (see also Part I., p. 23, et seq.), viz. :—

1. Monatomic or monobasic elements, Monads, one atom of which is in combination usually equivalent to H, or one atom of hydrogen. 2. Diatomic or dibasic elements, Dyads, one atom of which in combination is usually equivalent to H,, or two atoms of hydrogen. 3. Triatomic or Tribasic elements, Triads, one atom of which is in combination usually equivalent to H, or three atoms of hydrogen. 4. Tetratomic or Tetrabasic elements, Tetrads, each atom of which in combination is generally equivalent to H1, or four atoms of hydrogen.

5. Pentatomic elements, or Pentads, one atom of which usually represents five atoms of hydrogen, or is equivalent to five atoms of a monad.

6. Hexatomic elements, or Hexads, the atom of which is equivalent in combination to six atoms of hydrogen or some other monad.

The particular class of the element or radicle may, when needful, be pointed out as proposed by Odling, by affixing the marks ('') ('') () () () to indicate its dyad, triad, tetrad, pentad, or hexad character.

All the members of the same group exhibit a certain similarity in chemical characters; further, when any one of them enters into combination with any given element, A, the compound so formed is usually similar in atomic composition to the compounds

58

THEORY OF POLYBASIC ELEMENTS.

which the other members of the group form with the same given element, A. For example:

a. To the monad group are regarded as belonging-hydrogen, potassium, and the allied metals of the alkalies, and silver, as well as chlorine and bromine.

Each molecule of a monad consists of two atoms of the element united one with the other. Each of the monad elements may combine with a single atom of some other element of the same group; for instance:

b. To the dyad group the following elements are considered to belong:-Oxygen (=16), sulphur (S=32), and a large number of the metals, including those of alkaline earths, with those of the magnesian group, besides mercury, copper, and several others.

One atom of each of these elements may unite with two atoms of some one monad element, or with one atom of two different monads; for example:

The molecule of the dyad metals usually contains but one atom of the metal; whilst the non-metallic dyads contain two atoms in their molecule.

c. To the triad group belong boron, gold, and rhodium, and as still admitted by many chemists, nitrogen, phosphorus, arsenicum, antimony, and bismuth. One atom of each of these triad elements may combine with three atoms of an element of the monad group. As for instance :

d. To the tetrad group carbon (E12), and silicon (Si=28) belong, besides tin, titanium, zirconium, platinum, and some other elements.