II. SIMPLE BODIES OF CHEMISTRY.

I. HYDROGEN.

Hydrogen is so widely diffused in the material world, that it may well be regarded as one of the primary products of nature. It forms an essential constituent of the liquid matter of the globe, and exists in every production of the animal and vegetable kingdom. In the separate state, it is an aëriform fluid, at every known temperature, and under every degree of pressure. It is transparent, colourless, and without taste or odour. It is highly elastic, and the lightest of known bodies. Its specific gravity is .06896, so that 100 cubic inches of it weigh 2.1371 grains.

It is irrespirable, and an animal quickly perishes when confined in it. It does not support combustion, a lighted taper, when immersed in it, being instantly extinguished ; but it is itself inflammable, and its combination with oxygen is attended with the evolution of heat, and, in certain conditions, of light. It does not combine with oxygen at low temperatures; but if it be kindled in common air by the application of an ignited body, it burns tranquilly with a feeble light. If the two gases, however, be previously mixed together in certain proportions, and if a body heated to bright redness be applied, or if an electric spark be passed through the mixture, the gases combine on the instant with a flash of light, and loud explosion. In these cases they combine with detonation; but when they are heated to a high tem

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perature without ignition, they combine slowly, and without explosion.

When a stream of this gas is directed on a piece of platinum perfectly clean on the surface, or in a minute state of division, the metal becomes instantly incandescent; and if oxygen gas or common air be present, the hydrogen gas is inflamed; and if the mixture of gases be nearly in the proportion to form water, detonation takes place. It would appear from this action, that platinum condenses the gas by contact, and that the sudden evolution of the latent heat of the hydrogen, is sufficient to raise the metal to the state of incandescence.

An enormous quantity of heat is evolved by the combination of this substance, and the most intense temperature which has yet been caused by artificial means, is produced by its combustion. This effect is applied to use in the oxyhydrogen blowpipe.

The same action that produces heat, produces the most. brilliant light that yet has been produced by art. When the jet of the blowpipe is thrown upon a mass of quicklime, the earthy body is heated to a degree of great intensity, and produces a light so white and vivid as to dazzle the eye. When the heated object is placed in the focus of a parabolic reflector, the light is reflected to an incredible distance.

Hydrogen gas being the lightest of all known bodies, it is exceedingly convenient to adopt it as the standard by which the combining weight of other bodies is estimated; and it is remarkable that chemists should have preferred taking oxygen for such a standard, in place of a substance so much better suited for the ends of easy comparison.

Hydrogen combines with many bodies, and produces remarkable compounds, one of which is the protoxide, or

water.

Water, unlike the substances of which it is composed, assumes the conditions of solid, liquid, and aëriform, within the range of ordinary temperatures. It is a powerful refractor of light, and an imperfect conductor of electricity.

Pure water is, by the universal consent of philosophers, assumed to be the standard with which the weight of all solid and liquid bodies shall be compared. A cubic inch of it at 62° F., or 16° C., and under the pressure of 30 inches of the barometer, weighs 252.458 grains, and is, therefore, about 818 times heavier than atmospheric air.

Water, from its nature, and the vast range of its affinities, is one of the most necessary agents possessed by the chemist. Innumerable changes are produced by the decomposition and recombination of its elements; and it is the most general solvent in nature. When it combines in definite proportions, it follows the general law of chemical combinations, and its combining weight is the sum of that of its two elements. It has all the essential characters of an oxide, and may be regarded as the type of the class.

It has the property of absorbing and dissolving various gases, in a greater or smaller proportion, when its surface is in contact with them. Thus it absorbs and dissolves from about 3 to 6 per cent. of its volume of oxygen gas, and about 100 per cent. of its volume of carbonic acid; and this property fits it for the support of animals that live in water, and for the nourishment of plants, whether under water or on land.

Water combines with a further proportion of oxygen, and forms a remarkable compound. The binoxide or peroxide of hydrogen is a colourless transparent inodorous liquid, of the specific gravity of 1.452, that is, it is 45 per cent. heavier than water. It preserves its liquid form at every degree of cold to which it has yet been subjected; but it is decomposed at a temperature of 59° F., being then resolved

into water, and oxygen gas, and when exposed to the temperature of 212°, it is decomposed with explosion.

It is powerfully acted upon by most of the metals, and many of the metallic oxides, which decompose it, but with different effects. Some of the metals are themselves oxidated, combining with the disengaged oxygen of the peroxide. These are metals which have a considerable affinity for oxygen. Some of the metallic oxides, on the other hand, lose the oxygen with which they were before combined, which passes away along with that of the peroxide. Thus the oxides of gold, silver, lead, mercury, and platinum, metals whose affinity for oxygen is feeble, are decomposed the instant they are brought into contact with the peroxide.

This substance whitens the surface of the skin, and after a time destroys its texture. It acts on vegetable colours, and renders them white. In this and other respects, it resembles the peroxide of sulphur, so that oxygen and sulphur resemble one another in this respect, that, combined in a certain proportion with another body, they exercise a similar action.

Hydrogen combines with sulphur, chlorine, phosphorus, and other allied bodies, and forms a series of compounds which exhibit a common class of properties, and connect together many of the bodies termed simple.

Hydrogen combines with carbon, and forms an extensive group of compounds, chiefly derived from the organic kingdoms, as olefiant gas, naphtha, naphthaline, and many more. The basis of all these bodies may be regarded as HC.

Hydrogen possessing the lowest atomic weight of any known substance, we cannot suppose it to be resolved into any other body known to us. The molecules of hydrogen may therefore be supposed to be more simple than those of other bodies; but it does not follow that the molecules of