substances to which it can impart the acid character are those which Lavoisier calls acidifiable or salifiable bases; of the simple substances these are, nitrogen, phosphorus, sulphur, carbon, the muriatic radical, together with a few of the earths and metals *.

Oxygen, however, is not always combined in the same quantity with the same matter; whence a distinction is made by the modern chemists, marked by the terminations ous and ic. Thus sulphurous, nitrous, or phosphorous acid mean the substance combined with a smaller proportion of oxygen; and sulphuric, nitric, or phosphoric acid mean the combination carried to the highest point of saturation.

From this distinction with respect to the acids arises another, which relates to the compound salts into which they enter. Thus the compound salts formed by those acids which terminate in ous are marked by the termination ite. Thus we have sulphites, phosphites, &c. Those which result from a combination with the acids ending in ic, or the full or strong acid, are marked by the termination at. Thus we have sulphats, phosphats, &c. &c.

Oxygen, we have also seen, combines with the metals, destroying the metallic lustre, and giving them an earthy appearance. This product was formerly called a calx, or cinder; but

It will be hereafter seen that oxygen is not the only acidifying principle.

this term we now reject, because it implies that it is always the product of combustion, whereas oxides are produced equally by the solution of the metals in the acids, and afterwards precipitating them by an alkali.

Oxygen combines with the metals in different proportions, and these products are marked by modern chemists by the Greek numerals. The protoxide means the metal combined with the smallest portion of oxygen, deutoxide with a still larger portion, and peroxide the largest it is capable of receiving.

Some of the metals become oxidized hy mere exposure to the damp and moisture. In most cases, however, heat is necessary; and the perfect metals, as they were formerly called, gold, silver, and platinum, will not oxidize by heat alone, but require the agency of an acid, from which they may be precipitated by an alkali in the state of an oxide.

All oxides are heavier than the quantity of the metal which produced them in proportion to the quantity of oxygen with which they are combined.

III. NITROGEN derives its name from being the basis or constituent of the nitric acid. This is proved by an easy experiment. If a tube is inverted in mercury in the pneumatic apparatus (Plate VI. fig. 19.), and filled with thirteen parts of nitrogen gas and eighty-seven parts of oxygen gas, and an electric spark is made to pass through

this mixture of the gasses, they will be condensed, and form pure nitric acid.

*

Nitrogen has not been made apparent to our senses otherwise than in a state of combination. The simplest form is that of a gas, or combined with caloric. In this state it forms about 77 parts in the 100 of our atmosphere, and is that part of the common air which will neither support flame nor animal life. On this last account it was formerly known by the name of azote or azotic gas, because it was destructive of animal life. As it constitutes a part of the atmospheric air, and is incombustible, it is easily procured by abstracting that part of the air which is destroyed by combustion, viz. the oxygen gas. Thus, if a portion of iron filings and sulphur, moistened with water, is put into a vessel with common air, the oxygen will be absorbed, and will go to oxidize the metal and the sulphur, and the nitrogen gas will remain. The same effect will be produced by phosphorus alone, inclosed in a similar vessel with common air. The phosphorus will undergo a slow combustion, will be oxidized, or converted into phosphoric acid, and the nitrogen gas will remain behind.

Nitrogen gas is considerably lighter than common air, and consequently much lighter than oxygen gas. Its specific gravity is to that

* From the Greek privative « (a or ab, Latin), and zoe, life.

Com

of common air nearly as 942-6 to 1000. pared with hydrogen its specific gravity is as 13

to 1.

It is immediately destructive of life if breathed without a mixture of oxygen gas; though it may be doubted whether the animal does not rather die from the want of oxygen gas. than from any noxious quality in the nitrogen; since, when mixed with the former, every moment's experience convinces us that nitrogen gas is not only harmless, but even salutary. Its use in the atmosphere appears indeed to be to dilute the oxygen, and prevent its too violent effects on the animal frame.

Nitrogen unites with oxygen, besides its combination forming atmospheric air, in four different proportions, constituting nitrous oxide, nitric oxide, nitrous acid, and nitric acid.

Nitrogen gas extinguishes flame for the same reason that it extinguishes life, namely, from wanting that principle which supports bothoxygen. From this circumstance we may see the absurdity of the common practice when a room or a house is set on fire, which is, to open all the doors and windows, by which a constant supply of fresh air is produced, so as to enable the fire to burn with greater violence.

In the gaseous form there are two combinations of nitrogen with oxygen, which it is proper to notice from their singular properties. The one is that to which one of the first chemists

of the present day (Sir Humphry Davy) has given the name of the nitrous oxide. It may be taken into the lungs by breathing, to a certain degree, with no dangerous consequences. But the effect is a momentary intoxication with exceedingly pleasurable sensations. The effects, however, cease in a very little time, and leave no sensation of languor behind, as in other cases of intoxication. The proportion of the ingredients, is supposed to be about seven parts of oxygen to thirteen of nitrogen.

The other is that which is called nitrous gas, and which is merely an unsaturated combination of nitrogen with oxygen. Hence it has a very strong attraction for oxygen, which it takes even from the atmospheric air. When brought into contact with common air, therefore, it rapidly absorbs the oxygen from it, forming nitric acid, and consequently destroys a certain portion of the mass. Hence it was employed as a test of the purity of air (more properly of the quantity of oxygen it contained), and an instrument was constructed called the eudiometer. This was only a glass. tube, such as that exhibited in the pneumatic apparatus, fig. 19. In this was included a certain portion of any air which was meant to be tried; to this nitrous gas was admitted; and in proportion as the air in the tube was lessened (that is, in proportion to the diminution of the oxygen), the air was supposed to be more or less pure. Nitrous gas is estimated,