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Bodies which in combining with Hydrogen lose nothing of their

acid character.

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SUBDIVISION II. OXYGEN, SULPHUR, SELENIUM, TELLURIUM. All these substances are known, and can be easily isolated; the two former in fact occur in great abundance, and nearly pure in nature. They are very slightly inferior to the former group in their power of uniting with the basic elements, and when thus combined, they form compounds which are difficult of decomposition. The first member of this subdivision, oxygen, is a gas; the other three are solids, at common temperatures. As a family, they bear almost as strong a resemblance to each other as do the members of the first subdivision. One remarkable feature, however, must be noticed with regard to oxygen and sulphur, the two most abundant elements of this group,-bodies whose properties have been very thoroughly investigated. These elements enjoy the power of combining, not only with basic elements, but also with nearly all bodies of their own class: oxygen, e.g., forms chemical compounds with every known simple substance except fluorine, and when it combines with an acid element, the resulting compound, as might naturally be expected, possesses in a marked degree the characteristic acid properties of its two constituents.

I. OXYGEN=0. The non-metallic element, or salt-radical oxygen, is a gas at all known temperatures and pressures.

* V.=vapour; L.=liquid; S.=solid.

The great reservoir of this gas is the atmosphere, of the total volume of which it constitutes one-fifth ; and by reason of its power of combining with every other known element, fluorine only excepted, it is also an abundant constituent of the earth's crust. It is the presence of this substance which fits the air for the support of animal life: the pure gas inhaled into the lungs does not prove poisonous.

The simplest method of isolating this element is by heating some compound in which it exists not very closely combined, for the action of heat is often one of decomposition; and thus if we select a combination of mercury (Hg) with oxygen (O), oxide of mercury (Hg,0) in fact; or of silver (Ag) with oxygen (O), oxide of silver (Ag,0); we shall find that these bodies will split into metal on the one hand, and into oxygen gas on the other, by the simple elevation of temperature. If we employ a still higher degree of heat, other oxides, such as that of manganese (Mn, 0,), will be found to part with a portion of their oxygen. But the most usual method is to heat the substance known as chlorate of potassium (KCI O), which is a compound containing the basic element potassium (K), united to an acid body called a compound salt-radical, and composed of two acid elements, namely, chlorine (CI) and oxygen (O), in the proportion of three equivalents of oxygen to one equivalent of chlorine. This compound is called anhydrous chloric acid, or chloric anhydride. When chlorate of potassium (KCI 0,) is heated, the 30 are expelled, and the compound called chloride of potassium (KCI) remains, according to the following equation

KCIO,=KC1+30. This is the method which the student should employ in the preparation of this element. The following properties of oxygen gas are to be noticed :

(a) It is without colour and odour, and does not perceptibly dissolve in water.

(3) It does not exert a bleaching power like chlorine.

(y) A taper or match just blown out, but with the wick still glowing, is immediately rekindled when plunged into this gas,

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

(8) 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

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,

(B) Its power of combining, when heated, with the atmospheric oxygen.

() 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

Hydrogen.

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SUBDIVISION III. 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 ;

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