necessary to respiration and combustion. By the process of combustion, or of breathing, carried on in a confined vessel, this gas will be abstracted, and the remaining gaseous matter is

2. AZOTIC or NITROGEN GAS. It is colourless and elastic. It stops combustion, and destroys animal life, when breathed in a pure state. From the union of azote with oxygen, in the proportion of 30 to 70, an acid is formed, which is termed nitric acid. This combination may take place with three different doses of oxygen. When the dose of oxygen is complete, NITRIC ACID is formed, being a yellow and highly corrosive fluid: it acts with great power on many other bodies, and parts with its oxygen with great readiness. That part of the acid which thus loses a portion of its oxygen escapes in a gaseous form, and is then called NITROUS GAS, consisting of nitrogen united with a diminished portion of oxygen.

On mixing with the atmosphere, it assumes the appearance of a yellow vapour, from its union with the oxygen of the atmospheric air, and with the moisture it may contain. By this acquisition of oxygen, it may again become nitric acid.

A combination, in which the proportion of oxygen to azote is still lower, is formed by allowing nitrous gas to stand for some days over iron filings. It will thus be materially changed. It no longer becomes yellow on being mixed with atmospheric air, and it now supports combustion, so that bodies will burn

in it with a considerable degree of splendour. It is now called the gaseous oxide of azote. The salts formed from this acid are termed nitrates.

Atmospheric air, it should be remembered, is constantly formed, in every situation, of certain and constant proportions of the oxygen and azotic gas. It also contains generally a little more than a thousandth part of carbonic acid gas, of which we will next treat.

If charcoal be heated in a closed vessel of oxygen gas, it takes fire, and the whole is consumed. The bulk of the gas remains the same, but part of it is changed to a gas of a different kind. It will not support flame, and is destructive of animal life, on being respired. Lime-water absorbs it, and is thereby rendered of a milky hue. It hence appears that a peculiar substance, the chief constituent of the charcoal, has by the process of combustion entered into combination with the oxygen, and has formed an acid gas of a peculiar kind.

The substance yielded by the charcoal, and which appears to have been its chief constituent, is termed CARBON, and the acid gas which it forms by this union is termed, 3. CARBONIC ACID GAS. This gas

is obtained in nearly the same manner from the burning of the diamond, or of plumbago, commonly called black-lead. The salts formed by this acid are termed carbonates.

WATER is formed by the union of oxygen with another gaseous substance.-4. HYDROGEN.

Its

existence in water is thus manifested. Water in a state of vapour being made to pass over iron wire made red hot, the oxygen of the water combines with the iron, and the water disappearing, the hydrogen gas remains. Being mixed with half its

bulk of oxygen gas, and fired with the electric spark, the mixture explodes, the gases disappear, and water is formed. It is twelve times as light as common air, and when properly confined, as in a balloon, rises through the air, as a bubble of common air does through water. Mixed with a small portion of oxygen, it burns with brilliancy, as when united with the oxygen in the common air, in what are termed the gas-lights; but with a larger proportion of oxygen it explodes. It enters into the formation of many animal and vegetable substances, from which it is evolved during their decomposition. It also exists in many mineral substances, from which it is separated by different processes.

From stagnant water, luminous and other vapours are evolved: among these a combination of carbon and hydrogen may be first noticed. It was formerly termed heavy inflammable air, but is now named CARBURETTED HYDROGEN GAS. It is highly inflammable, and burns with a yellow brilliant flame; and, under certain circumstances, fires with an explosion.

From these naturally formed vapours becoming spontaneously luminous, it is very likely that a portion of phosphorus may sometimes enter into their

composition. This is rendered probable, from its being well known that a phosphorescent light is sometimes yielded by the natural decomposition of urine, and of other animal substances; independent of those luminous appearances unconnected with the presence of phosphorus, as in the instances of decaying fish, rotten wood, &c. It is, indeed, far from being improbable, that in the laboratory of nature, where decomposition and reformation are proceeding on so large a scale, and in so powerful a degree, the hydrogen gas may become impregnated with phosphorus, and form the PHOSPHURETTED HYDROGEN of the chemists; and that this may concur in the formation of ignes fatui, or will of the wisps, &c.

To enable our readers the better to judge of this question, we will give a brief history of this substance. PHOSPHORUS is a yellowish, semi-transparent substance, which, when exposed to the air, emits a white smoke, smelling like garlick. If heated to 148° it takes fire, and burns with a brilliant white flame, and abundance of white smoke. This smoke being collected, is found to be a peculiar acid, termed PHOSPHORIC ACID, having been formed by the accession of oxygen to the phosphorus, during its combustion. But the oxygen may be combined with the phosphorus in a less quantity, and thereby form the oxide of phosphorus. Its compounds with other bodies are termed phosphurets; and the gas to which we referred in the former paragraph, being a compound of hydrogen with phosphorus, is termed phosphuretted hydrogen gas,

b

and flames spontaneously on coming in contact with the atmospheric air. The salts formed with this acid are termed phosphates.

Agreeing so nearly in its combustibility with the substances just treated of, SULPHUR or brimstone may be very properly examined in this place. It is of a yellow colour, hard and brittle, and of but little taste. It yields its peculiar smell on being very little heated. At 170° it sublimes in a fine powder, termed flowers of sulphur; at 218' it melts, and at 560° it boils; the vapour kindles, and if the fumes are collected, a ponderous acid liquor is obtained, which is the sulphuric acid. When the acid is obtained under circumstances occasioning a less portion of oxygen to combine with the sulphur, the product is termed sulphurous acid. The salts in the former case are termed sulphates, and in the latter, sulphites.

It readily combines with hydrogen gas, forming sulphuretted hydrogen gas, which is colourless, and smells like rotten eggs. When an additional portion of sulphur is forced into this compound, supersulphuretted hydrogen is formed.

Sulphur readily combines, too, with phosphorus, forming a compound which is very combustible, and often takes fire spontaneously. As this compound may be formed in different proportions, its appellation varies accordingly. If the proportion of sulphur is largest, it is termed phosphuret of sulphur ; and if the phosphorus exceeds, sulphuret of phosphorus.

We must here notice another combustible sub