These terms are very appropriate, since they are all derived from the same root as our common English word equivalent, which best expresses the fundamental idea which underlies the whole subject."

The quantivalence, whenever important, may be indicated by a Roman numeral placed over the atomic symbols; thus,

5. Atomicity. "The quantivalence of an element is shown, not only by its power of replacing hydrogen atoms, but also by its power of replacing any other atoms whose quantivalence is known. Moreover, what is still more important, the quantivalence of an element is shown not only by its replacing power, but also by what we may term its atom-fixing power; that is, by its power of holding together other elements in a molecule. We may take as examples the molecules of four very characteristic compounds; namely, hydrochloric acid, water, ammonia, and marsh-gas," whose symbols are HCl, H2O, H2N, and HC.

66

II

III

By these symbols it appears that, while the univalent atom of chlorine can hold but I atom of hydrogen, the bivalent atom of oxygen holds 2, the trivalent atom of hydrogen 3, and the quadrivalent atom of carbon 4 atoms of the same element."

"The quantivalence of the chemical elements, especially as indicated by their atom-fixing power, is by no means always the same. That of the univalent elements alone is constant. They always combine with single atoms. But the other elements frequently exhibit, under different conditions, an unequal atom

fixing power. Thus we have SnCl2 and SnCl4, PCl, and PC15,

III

NH, and ÑHCI. Each element, however, has a maximum

power, which it never exceeds. This we shall call its atomicity; and we shall distinguish the elements as monads, dyads, triads, etc., according to the number of univalent atoms they are able at most to bind together. Thus nitrogen is a pentad, although it is commonly trivalent; and lead is a tetrad, although it is usually bivalent."

The prevailing quantivalence of an element is a more charac

teristic property than its atomicity. The former is generally well established, while the latter is frequently still in doubt. 6. Quantivalence of Radicals.- "When in the molecule of any compound the dominant or central atom is united to as many other atoms as it can hold of that kind, the molecule is said to be saturated. Thus HCl, H2O, HÖN, and HC are all saturated

molecules. On the other hand, the molecules CO, PCl, and

II

SnCl2 are not saturated; for they can combine directly with more oxygen or chlorine, forming thus the saturated molecules CO2, PCl5, and SnCl4." It will be seen that CO, PC13, and SnCl, are bivalent radicals, since they can each hold the equivalent of two atoms of hydrogen. The CO can hold an additional atom of oxygen, and PCl, and SnCl2, can each hold two additional atoms of chlorine. The quantivalence of an atom of C is 4, and that of an atom of O is 2; and the difference of these is 2, the quantivalence of the radical CO. In general, the quantivalence of a binary radical is the difference of the quantivalences of its elementary atoms. Thus, the quantivalence of N is 5, and that of O2 is 4; and that of the radical NO2 is 1. The quantivalence of SO2 is 2, that of S being 2, and that of O, being 4. The quantivalence of PO is 3, that of P being 5, and that of O being 2.

The quantivalence of CIO, is I. This radical illustrates the way in which elements often seem to combine. The 2 atoms of O unite with each other, and thus neutralize 2 of their equivalents, leaving 2 unsaturated. Now I of these is neutralized by the Cl, leaving I unsaturated. Thus CIO, becomes a univalent.

CHEMICAL TYPES.

7. Types of Chemical Compounds. "There are three modes or forms of atomic grouping to which so large a number of substances may be referred, that they are regarded as molecular types or patterns, according to which the atoms of a molecule are grouped together. . . The three compounds, hydro

chloric acid, water, and ammonia, HCl, H2O, and H&N are generally taken as the representatives of these types; and substances are said to belong to the type of hydrochloric acid, to

the type of water, or to the type of ammonia, as the case may be. . . . It will be noticed that in the first of these types a single univalent atom or radical is united to another single univalent atom; that in the second, a bivalent atom binds together two univalent atoms, or their equivalents; that in the third a trivalent atom binds together three univalent atoms, or their equivalents."

8. Condensed Types. "In the same way that a bivalent atom may bind together two univalent atoms or their equivalents, so also it may serve to bind together two molecules; and, in like manner, a trivalent atom may bind together three molecules into a more complex molecular group; and thus are formed what are called condensed types."

9. Metallic Chlorides. —All the metallic chlorides, fluorides, bromides, iodides, and cyanides may be considered as belonging to the muriatic acid type. In those of the monad metals, there is no condensation, or soldering together of molecules. Thus we have

HCl (muriatic acid).

AgCl (argentic chloride).

Here one atom of hydrogen in a molecule of HCl may be regarded as replaced by an atom of a univalent metal.

The chlorides, etc., of the bivalent metals may be regarded as formed by the soldering together of 2 molecules of HCl by a bivalent atom which replaces the H. Thus,

HC1

HCI

(two molecules of muriatic acid).

Ca Cl} = CaCl, (calcic chloride).

The chlorides, etc., of the trivalent metals may be regarded as 3 molecules of HCl, soldered together by a trivalent atom, which has replaced the H. Thus,

The chlorides, etc., of the quadrivalent metals may be regarded as four molecules of HCl, soldered together by a quadrivalent atom which replaces the H. Thus,

The symbol of aluminic chloride is Al1⁄2Cl. Here, although Al is quadrivalent, Al1⁄2 is only sextivalent, not octivalent. Two atoms of Al seem to combine with each other, and thus neutralize 2 of their 8 equivalents. There are several cases like this. Chromium, iron, manganese, uranium, and some other metals are both bivalent and quadrivalent. When quadrivalent, they combine like aluminium. Thus we have

Cr2Cle, chromic chloride.

Fe,Cl, ferric chloride.

Mn,Cl, manganic chloride.
U2 Cly, uranic chloride.

As bivalents, they form the following chlorides : —

CrCl2, chromous chloride.

FeCl2, ferrous chloride.

MnCl2, manganous chloride.
Uch, u ranous chloride

21

The bivalent mercury forms the two chlorides, HgCl2, mercuric chloride, and Hg2Cl2, mercurous chloride. In the last, the 2 atoms of Hg combine with each other, and thus neutralize 2 of their 4 equivalents, leaving the other 2 to be neutralized by the Cl2. The bivalent copper is like mercury in this respect.

10. Bases. The bases belong to the water type, and may be regarded as made up by replacing a part of the H of the H2O by an equivalent of a metallic element. In the bases of the univalent metals, I atom of H is replaced by 1 of the univalent metal. Thus,

The bases of the bivalent metals are of a condensed type, 2 molecules of H2O being soldered together by a bivalent atom, which replaces an atom of H in each. Thus,

H2H2O2 = 2H2O

CaH2O2 = calcic hydrate.

The bases of the trivalent metals are also condensed in type, 3 molecules of H2O being soldered together by a trivalent atom. Thus,

The quadrivalent aluminium forms a base by soldering together 6 molecules of H2O with 2 trivalent atoms. Thus,

It will be seen that this hydrate is analogous to the aluminic chloride already explained. In the same way, chromium, iron, manganese, and uranium, as quadrivalents, form the hydrates

Cr2HO, chromic hydrate.

Fe2HO, ferric hydrate.
Mn2H6O6, manganic hydrate.

U2H6O6, uranic hydrate.

As bivalents, they form the bases

CrH2O2, chromous hydrate.

FeH2O2, ferrous hydrate.

MnH2O2, manganous hydrate.

UH2O2, uranous hydrate.

2

Mercury also forms two bases, HgH2O2 and Hg2H2O2, corresponding to its two chlorides.

11. Basic Oxides. - Basic oxides belong to the water type, and may be regarded as formed by replacing all the H of one or more molecules of H2O by an equivalent of metal.