EMPIRICAL AND RATIONAL FORMULA.

planation of the mode in which he conceives those elements to be associated together, and by the arrangement of his symbols may give expression to a theory of the constitution of the body, and thus assign to it a rational formula. A body can have but one empirical formula, but it may be represented by a variety of rational formulæ, according to the different views which may be taken as to the mode in which its components are arranged.

Crystallized sulphate of magnesium, for example, adopting the new notation, has the following empirical formula :—

(1) MgH14 SO11;

that is to say, its constituents are present in the ratio of 1 atom of magnesium, 14 of hydrogen, I of sulphur, and 11 of oxygen. It is, however, never so written. The water which it contains may be entirely driven off by a heat a little above 400° F.; and it is usually represented as consisting of magnesia, sulphuric anhydride, and water, as these are the materials out of which it is formed: thus—

But it is found that at a heat of 2120, 6 atoms of the water may be expelled, whilst the seventh atom requires a much higher temperature, so that it appears to occupy a position in the salt different from that of the other 6: this fact may be indicated by slightly altering the second formula, as follows:

(3) MgO, SO ̧ . H‚Ð,6 н¿Ð.

Many chemists, however, guided partly by the results of the electrolysis of the salt, suppose that when once an acid and a base have reacted on each other, their elements are arranged in an order different from that in which they existed when separate, and they prefer to represent the salt accordingly, as

(4) Mg,SO. H‚Ð, 6н‚Ð.

Each of the last three formulæ is a rational formula for sulphate of magnesium; and each conveys far more information than the formula No. 1. Each represents a theory founded upon particular modes of decomposition which the salt may be made to experience.

It is impossible that all these formulæ should truly indicate the molecular constitution of the salt, though each may represent the grouping of its component elements under particular circumstances. Rational formulæ are indeed indispensable as the exponents of the theories which guide the chemist in his researches, or which aid him in arranging and interpreting phenomena; but,

8

GENERAL ARRANGEMENT OF THE ELEMENTS.

like the theories which they represent, they are only temporary expedients, and they must consequently always be regarded as such, and must be modified or discarded when they no longer faithfully represent the conditions of our knowledge of the compounds which they are employed to indicate. A perfect rational formula would embrace all the modes of decomposition of which a compound is susceptible, and would represent the constitution of the body as well as its various analogies and relations.

(333) General Arrangement of the Elements adopted in this Work. The general division of the elementary bodies into nonmetallic and metallic has been already pointed out. There is, however, no strict line of demarcation between the non-metallic and the metallic elements.

The bodies which are considered as non-metallic constitute the electro-negative ingredient in the binary combinations which they form with the metals, and are most of them insulators of the voltaic current. Carbon and silicon, however, in certain forms, act as conductors of electricity. The compounds of the nonmetallic elements with oxygen generally show but little tendency to unite with acids; on the contrary, the higher oxides of most of them form compounds which, if acted on by water, furnish powerful acids. These acid-forming oxides, except silica, are readily soluble in water; and even silica, under certain circumstances, may be obtained in solution.

The metals, on the other hand, are characterized by a peculiar combination of opacity and compactness, which gives them, when polished, a peculiar brilliancy, termed the metallic lustre; they are good conductors of heat and electricity, and most of them, by combination with oxygen, form powerful bases. It is, nevertheless, sometimes difficult to determine whether a body should be regarded as a metal or not. Arsenicum has a high metallic lustre; but it is more closely allied to phosphorus than to any other elementary substance, and both its oxides, when dissolved in water, are endowed with well-marked acid properties. Tellurium also exhibits the closest analogy with selenium and with sulphur, but it possesses high lustre, and some conducting power for electricity.

The subdivision of the simple substances into non-metallic and metallic is, however, convenient to the student, and it will therefore be retained in this work. The order in which the different elementary bodies will be treated is not in all cases that which a rigid adherence to analogy would indicate, though this has been adopted, excepting in those instances in which it seemed more advantageous to the student to follow a different

GENERAL ARRANGEMENT OF THE ELEMENTS.

9

course. In most cases we shall first examine the chemical properties which are exhibited by each of the elements in its uncombined form; we shall then study the general nature of its actions upon other elements, and shall afterwards examine the more important compounds into the formation of which it enters. In describing the properties of the non-metallic elements, it will be found a convenient arrangement to consider first the four elements which enter into the composition of those all-pervading and all-important substances, air and water, and then to pass on to others, classing them together according to the general aualogy of their properties. Following this plan, we shall consider first the properties of what may be termed the atmospheric group of elements-viz., oxygen; nitrogen (and the atmosphere); hydrogen (and water); carbon (and carbonic acid).

We shall next examine some of the more important compounds which these bodies form with each other, and shall then describe the well-marked natural group constituting what Berzelius termed the Halogens, from the circumstance of their forming with the metals saline compounds resembling common salt; this group comprises chlorine, bromine, iodine, and fluorine.

Three combustible elements will be taken next in order-viz., sulphur, selenium, and tellurium: these will be followed by phosphorus, and the general survey of the non-metallic elements will be completed with silicon and boron.

For the convenience of description and of reference, the metals will be arranged in eight groups, in the following order. The elements which compose each group generally present some natural resemblance, though, as already stated, the classification does not in all cases bring together those which, in chemical habitudes, are really the most closely allied.

I. Metals of the Alkalies—5 in number.

I. Potassium.

2. Sodium.

3. Lithium.

4. Cœsium.

5. Rubidium.

II. Metals of the Alkaline Earths-3 in number.

V. Metals more or less analogous to Iron-6 in number.

1. Cobalt.

2. Nickel.

VI. Metals which yield Acids—10 in number.

5. Molybdenum.

6. Vanadium.

8. Arsenic.

9. Antimony. 10. Bismuth.

5. Chromium.

If a strictly natural order were to be followed in grouping the elements, it would, however, be necessary to modify the foregoing arrangement. This will be rendered evident by pointing out the most important natural groups into which the elementary bodies admit of being subdivided. The detailed indication of the points of resemblance between the members composing each group must be deferred until the properties of the group are considered. In many instances these natural relations between the individual elements thus grouped together are very striking, in others they are more obscurely marked, and in the case of the metals of the earths proper, as well as of the noble metals, the natural chemical relations of these elements with the others are as yet but incompletely known. In the table which follows, the principal elementary bodies are represented in two converging series.

FIRST DIVISION.-NON-METALLIC ELEMENTS.

THE following table gives a general view of some of the most important of the constants of the non-metallic elements. The squares employed in the column headed " Atomic Volume," indicate the relative volumes occupied by a quantity in grains of each of the different elements, corresponding to the numbers given in the column of "Atomic Weights," assuming that the space occupied by one grain of hydrogen under similar circumstances of temperature and pressure, would fill a space of one square or one volume.