carbon, however, is an exception, as it has a very powerful tendency to unite with hydrogen, a union which takes place in many proportions; it also combines with the metals. They all combine with oxygen, like the members of the two preceding classes, to form compound salt-radicals or acid bodies. In many of their characters these three elements bear a strong family resemblance to one another. They are all solids. The remaining four elements of this group are of very rare occurrence.

I. CARBON=C. The non-metallic element, or salt-radical carbon, is a solid at ordinary temperatures.

There is no element which enters into so large a number of compound bodies as carbon. It forms a great part of the organic world, the animal and vegetable kingdoms; and organic chemistry includes the history not only of all the compounds of carbon occurring in nature, but also of a countless host of such compounds produced from time to time by artificial aid. Carbonic acid (CO2), a combination of carbon and oxygen, is the great reservoir of this element; and just as marine plants withdraw iodine (I), as we have seen, from sea-water, so does terrestrial vegetation remove carbonic acid (CO2) from its reservoir the atmosphere (in which, however, it exists in small proportion), decomposes it, and accumulates the carbon in various forms of combination with hydrogen, nitrogen, and oxygen. Since many animals live on plants, and carnivorous animals prey on the herbivorous, the whole organized world is thus supplied with the necessary carbon. The carbon is restored to the atmosphere by various processes of civilized life, but especially by the respiratory process going on in the animal organism. The great deposits of coal which occur in many parts of the earth's crust are chiefly combinations of carbon with hydrogen, but they also contain some nitrogen, oxygen, and small quantities of other bodies; they result from the gradual decomposition of vegetable matters out of contact with air.

Carbon, as it usually occurs, is infusible, inodorous, and cannot be distilled. It exhibits allotropy in a more marked degree even than sulphur or selenium, occurring in three very distinct modi

fications. The first form, known as diamond, is transparent-the hardest of all bodies-and magnificently crystalline, the crystals, which are octahedra, having a density of 3·35: in the second condition carbon presents itself in the form of hexagonal plates, of bluish-black metallic lustre, soft and unctuous to the touch, with a specific gravity of 1.9 to 2.3; this second form is known as plumbago, black-lead, and graphite; while the third variety is densely black, without lustre or any trace of crystalline structure; it occurs in numerous varieties of anthracite, charcoal, jet, and gas-coke, and in soot, lamp-black, &c. In this third form carbon has a specific gravity varying greatly in different cases. The first variety, or diamond, is termed a-carbon; the second, ẞ-carbon; and the third, y-carbon.

The peculiar characteristics of the different kinds of carbon mentioned above are well known: the great brilliancy of the diamond, the unctuous feel of graphite,-the rich black and considerable hardness of jet, anthracite, or cannel coal,-the dense, heavy and compact structure of the coke from gas-retorts,-the very light spongy texture of wood-charcoal, and the pulverulent lamp-black, are familiar to every one. There is, however, one property which the student should observe with regard to woodcharcoal especially, viz. its power of absorbing and retaining gases; one volume of this substance absorbing of ammonia, for instance, 90 volumes; of carbonic acid, 35; and of oxygen, 9 volumes.

As a chemical agent carbon has little power, possessing as it does, but a slight tendency to combine with other elementary bodies, oxygen and sulphur excepted. The chemist has therefore little influence in effecting artificial combinations between carbon and other bodies directly. Nevertheless there is no element which plays a more active or universal part in the chemistry of nature than carbon.

II. BORON=Bo. Of the non-metallic element, or salt-radical boron, but little was known until recently. It resembles carbon in being tasteless and odourless, and in being, if at all volatile, but doubtfully so. Boron differs from carbon by being soluble to a slight extent in water, and by not forming compounds with

hydrogen, hydrogen and oxygen, or hydrogen, oxygen, and nitrogen (N), in the same way as carbon. It occurs in nature combined with oxygen. Boron, like carbon, may be obtained in three distinct forms. The first variety, or diamond of boron (a-Bo), is a brilliant and extremely hard substance, crystallizing in acute octahedra; the second resembles graphite or plumbago most closely in its appearance, and is termed graphitoidal boron, or (B-Bo), while the third variety has no trace of crystalline structure, but is a soft brown powder (y-Bo).

III. SILICON=Si. The non-metallic element, or salt-radical silicon, has been recently investigated. In most of its properties it resembles boron, and like that element, occurs in three allotropic forms, the octahedral, the graphitoidal, and the amorphous.

IV. The other four bodies which are placed in this group, on account of a certain resemblance which they bear to silicon in some physical and chemical characters, are of very rare occurrence; titanium is met with more frequently than the others. They stand, together with vanadium, molybdenum, tungsten and arsenic, on the neutral ground between the basic and acid elements.

Bodies which lose their acid character completely by combination with Hydrogen.

* D. refers to the diamond form of the element; G. to the graphitoidal;

A. to the amorphous.

SUBDIVISION IV.

NITROGEN, PHOSPHORUS, ARSENIC, ANTIMONY.

These four bodies form a very peculiar group, the members of which are linked together by close chemical similarities, although differing greatly in their physical character: nitrogen, for instance, is a gas which has never been condensed; phosphorus is a solid occurring in two allotropic forms; and arsenic is a substance frequently classed among the metals. For the true metallic elements these substances have little affinity, although they do combine with some of them, especially with the more energetically basic; but for hydrogen they have a greater chemical attraction, uniting with three equivalents of that substance to form gases which partake in a greater or less degree of the basic character of the hydrogen. Perhaps the most striking compounds they yield are those with oxygen, with which element these radicals unite in several proportions; the combinations which contain three or four equivalents of oxygen being most energetic compound acid-radicals.

I. NITROGEN=N. The non-metallic element, or salt-radical nitrogen, is a gas at all known temperatures and pressures.

It forms four-fifths of our atmosphere, in which it is commonly said only to subserve the end of diluting the oxygen, and preventing it thus from exerting too violent an action on the animal respiratory organs, and the processes of combustion which are continually taking place upon the earth's surface; but it is probable that it plays another very important part in nature's economy, and that the vegetable world has the power of withdrawing it in some manner from the atmosphere, and consolidating it in the various organisms in which it occurs combined with carbon, hydrogen, and oxygen.

This element is prepared directly from the air, in which it exists merely mechanically mixed, not chemically combined, with oxygen, by removing the latter; this is done by the introduction of a substance capable of combining with the oxygen, leaving meanwhile the nitrogen unabsorbed; there is no difficulty in

finding such an agent. If the metal copper (Cu), e. g., be heated in a tube and atmospheric air passed over it, the copper withdraws the oxygen, so that the gas

collected at the delivery-tube is pure

nitrogen. Or, again, if a portion of air be confined in a jar over water and a piece of sulphur (S) or phosphorus (P), contained in a cup floating on the liquid, be ignited by a red-hot wire, the sulphur or phosphorus in burning combines with the oxygen, leaving the nitrogen free: the oxygen compound of sulphur (or phosphorus) formed is absorbed and dissolved by

Fig. 2.

the water. By this second method the student had better prepare some nitrogen, and notice

That any substance which burns in the atmosphere is immediately extinguished upon being plunged into this gas, simply because it is no longer in contact with an element with which it can combine.

The gas nitrogen has a specific gravity of 972; it is less soluble in water even than oxygen, water dissolving only 016 of its volume. Its chemical actions in the free state are so feeble, that nitrogen is commonly chosen as the type of chemical indifference; the elements for which it seems to have considerable affinity are potassium (K), sodium (Na), &c., hydrogen (H), carbon (C), and oxygen (O); but unlike most of the other elements which have passed under feview, nitrogen is not known to combine with any other element directly; the only exceptions to this behaviour are in the cases of titanium (Ti), and perhaps of boron (Bo). Titanium combines with nitrogen with the utmost avidity, precisely in the same way as the metals with oxygen or chlorine, &c.

=

II. PHOSPHORUS P. The non-metallic element, or salt-radical phosphorus, is a solid at the ordinary temperature.

Phosphorus, oxidized as phosphoric acid, is very widely dis