and burns with much increased brilliancy and rapidity; this is the most common test for the presence of oxygen.

(d) A piece of sulphur (S), ignited in the air, burns with much increased brilliancy when placed in this gas. The same effect is observed, though in a still more marked degree, in the case of phosphorus (P). Carbon (C), too, burns with great energy, and so also does the metal iron (Fe), when plunged while red-hot into a jar of oxygen.

The specific gravity of oxygen gas is 1.1057. It is slightly soluble in water, 1 volume dissolving ·035 of a volume of the gas; this very trifling solubility does not interfere with its being collected over water, which is by far the most convenient method of storing gases. In the free state it is not so energetic a chemical agent as chlorine and its congeners, at least at common temperatures, a few rare cases excepted; but when its temperature is raised, it combines with both basic and acid elements with perhaps greater energy than chlorine itself, and forms compounds of similar or nearly equal stability. These compounds are readily produced by heating the substance whose combination with oxygen we are desirous of obtaining, before their introduction into that gas; a union immediately begins, the temperature of the heated body is very much augmented, and a brilliant light is frequently kept up until the supply either of the oxygen or of the substance introduced is exhausted. Very striking effects are obtained by the combustion of heated iron, sulphur, carbon and phosphorus in this manner.

II. SULPHUR=S. The non-metallic element, or salt-radical sulphur, is a solid at ordinary temperatures.

This element is met with, uncombined, in extensive deposits which abound in volcanic districts; its great European source is Sicily. It is also largely diffused through the earth's crust in combination with various metals, and occurs in almost all waters in the form of sulphates, and in some springs as sulphuretted hydrogen (H, S). The great abundance of sulphur in the uncombined state renders it almost wholly unnecessary to extract it from any of its compounds; it is, however, sometimes ob

C

tained as a kind of bye-product in the manufacture of other substances.

Native sulphur is purified by simple distillation, and then sent into commerce. The student should observe with this substance : (a) Its physical properties, colour, fusibility, and volatility. (3) Its power of combining, when heated, with the atmospheric oxygen.

(7) Its power of combining with metals (such as copper [Cu]) when they are plunged into its vapour.

The specific gravity of sulphur in its ordinary solid form is 2.087; it melts at 120° C., and boils at 440°, becoming at that temperature a transparent gas of an orange colour, with a specific gravity of 6.654. Between, however, the temperatures of melting and ebullition, sulphur undergoes very peculiar changes of consistence, becoming up to the temperature of 260° C. more viscous, instead of more fluid, with each augmentation of heat. If sulphur thus heated be suddenly cooled, it presents none of the original features of the substance before fusion, as seen in the following comparison. Ordinary sulphur is yellow, this is brown ordinary sulphur crystallizes well either in acute rhombic octahedra or in long rhombic prisms, while this presents no trace of crystalline structure: ordinary sulphur is extremely brittle, this is as elastic as india-rubber. The present instance is the first which has been hitherto brought under the student's notice of an element existing in two forms or distinct states, but some of the elements shortly to be considered exhibit this peculiarity even more distinctly. To this phenomenon the term allotropy has been assigned. There are several other allotropic modifications of sulphur in addition to those just described. As a chemical agent sulphur is less energetic than oxygen, but it must not be forgotten that the solid form in which it usually exists is an obstacle to powerful chemical action; this is shown by the fact that sulphur in the gaseous condition combines energetically with many metals, which become red-hot in the act of combination; it can be considered therefore only slightly inferior to oxygen in chemical energy.

III. SELENIUM=Se. The non-metallic element, or salt-radical selenium, is of comparatively rare occurrence; in most of its physical and chemical properties it strongly resembles sulphur. Its ordinary form is that of a reddish-brown solid, with a somewhat metallic lustre. It fuses at 100° C., and if heated beyond this point and suddenly cooled, yields a viscous allotropic modification similar to that of sulphur. Selenium boils at a low red heat, and is converted into a yellow vapour. In its chemical properties it resembles sulphur still more closely, if possible, than in its physical characteristics.

IV. TELLURIUM-Te. The non-metallic element, or salt-radical tellurium, is even rarer than selenium; it occurs in nature combined with certain metals. In its physical properties it assimilates closely to the basic elements, but in its chemical tendencies it is allied to sulphur and selenium. It is white with a metallic lustre, crystalline, brittle, and has a density of 6.2. It is rather more fusible than antimony, and volatilizes at a red heat.

Bodies, the acid character of which is masked by combination with

CARBON, BORON, SILICON, TANTALUM, NIOBIUM, PELOPIUM,

TITANIUM,

The first three bodies are all known in the isolated state; they are comparatively destitute of affinity for the basic elements;

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 (CO), 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 (CO) 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, B-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