ammonia now combines with the disengaged nitrous gas, and forms nitrous oxyd.

To experience its effects in breathing it, put about a gallon into a large bladder, or oiled silk bag, having a tube attached to it, of three-fourths of an inch in diameter. First, the common air must be expelled from the lungs, before the tube is received into the mouth, and the nostrils must be accurately closed with the hand. It must then be breathed backwards and forwards into the bag for a few minutes.

Nitric Oxyde is called also nitrous gas. This compound of oxygen and nitrogen cannot be obtained by direct combination, but by abstracting from nitric acid a portion of its oxygen, leaving the remainder in such proportion as to constitute nitric oxide.

When pure, it is not acid, and is void of colour. It is incapable of supporting the combustion of most bodies; nevertheless, phosphorus and pyrophorus burn in it. Nitrous gas is made by putting clippings or filings of copper into a retort with nitric acid, diluted with thrice as much water; red fumes will be given out, if the gas is suffered to escape into the air; but if collected in the pneumatic apparatus, the gas is colourless. In this process, the metal attracts the oxygen from the nitric acid, and becomes oxydated; the rest of the acid being deprived of a great portion of its oxygen, can no longer exist as acid; it therefore expands, becomes aëriform, and appears as nitrous gas.

When nitrous gas and oxygen gas are mixed together in a glass vessel, previously exhausted of air, they instantly unite, and form a reddish coloured gas, which has but half the volume of the two gases, and which is highly acid. This new compound is called nitrous acid gas.

Nitrous acid gas is very easily absorbed by water, rendering it a green, sour liquid.

When nitrous gas is mixed with atmospheric air, the same red fume appears, from the nitrous gas uniting to the oxygen of the atmosphere, leaving the nitrogen by itself. Hence this gas has been used for measuring the quantity of oxygen gas contained in air, and an instrument for this purpose has been called an Eudiometer. A tall glass tube, sealed at one end, is used for this purpose, filled with water, and inverted in the pneumatic apparatus. Send up a certain measure of the air to be examined into this tube, and mark the space which it occupies in the top of the tube. Then add to it a measure of nitrous gas, and observe the degree of diminution. By comparing how much different specimens of air will be diminished by the same quantity of nitrous gas, the relative quantities of oxygen in each may be estimated. This method of analysing atmosphere is not considered as very correct, and other modes are sometimes used, where there are fewer sources of inaccuracy.

Nitrous Acid.

Considerable uncertainty prevails respecting this acid. The yellow coloured fuming acid, to which this name has been given, appears to be only nitric acid holding nitrous gas in solution. The nitrous gas may be expelled from it by the application of heat, and then the nitric acid is left colourless.

But if nitrous gas and oxygen gas be mixed together, without the access of water, by introducing them into a vessel previously exhausted of air, an union takes place; the two gases diminish to half the volume, and an acid gas is produced.

This is the nitrous acid in a gaseous form. This acid gas is extremely absorbable by water, which is at first rendered green: then, as the absorption goes on, it becomes blue, and, finally, of an orange colour: by adding more water, it may be brought back to the green colour.

If dry nitrate of lead be distilled, an orangecoloured liquid comes over; which is considered as nitrous acid nearly pure.

Nitric Acid.

This acid is one of those which have been longest known to chemists. It is so named from nitre, from, which it was procured. The corrosive acid called aqua fortis is an impure and weak nitric acid: but it was long used before its analysis was known: this we owe to Cavendish.

Nitre, called also saltpetre, consists of this acid united with potass, and the process for procuring the acid depends upon decomposing this salt. For this purpose, some substance is added to the nitre that has an attraction for the potass sufficiently strong to overcome that of the nitric acid, and, consequently, to allow it to be expelled by heat. Sulphuric acid is used for this purpose. During its expulsion, however, the nitric acid suffers a partial decompo-. sition; for though it is nitric acid that exists in the salt, it is nitrous acid that condenses in the receiver in the form of orange fumes. This is the common nitrous acid of commerce.

To convert this into nitric acid, another process is necessary. By distillation, nitrous acid gas is driven off, leaving the nitric acid colourless. Or,

it is distilled on black oxide of manganese, which gives more oxygen to it.

Nitric acid is extremely caustic; that is, acts powerfully upon animal substances. It unites with the alkalis and earths; it oxidates all the metals except gold and platina; it thickens and blackens oils, converting them into a coal, or inflaming them, according to the nature of the oil, and the degree of the concentration of the acid.

The combinations of nitric acid with different bases are called nitrates.

When the nitrogen and oxygen gases are mingled together, they form a compound exactly resembling common or atmospheric air.

Atmospheric Air

Is, indeed, essentially composed of these two gases: and its analysis or decomposition has been one of the most interesting discoveries of modern chemistry. It is curious that one of the ingredients of this substance, so necessary to animal life, should, by itself, be highly deleterious.

It has been completely proved, that the air of the atmosphere is a compound body, formed by the mixture of oxygen gas and nitrogen gas. The first is the only one of them that supports combustion; and when combustion takes place in common air, it is this part that unites to the burning body, forming either an oxide or an acid. If mercury be heated in a given quantity of atmospheric air for some time, it will become changed into a red powder, which will weigh more than the mercury; the air will be found to be diminished in quantity, and

to be no longer capable of supporting combustion. The reason of this is, that the oxygenous part of the air has united to the metal, and converted it into an oxyde, leaving behind only the nitrogen.

This decomposition of the atmospheric air may be effected more easily by burning phosphorus in it. During the combustion of the phosphorus, it unites to the oxygen, and forms phosphoric acid; the remainder is nitrogen.

The proportion of oxygen gas contained in a given quantity of atmospheric air can be ascertained by various processes. One method is, by inverting a glass tube into a solution of sulphuret of potass. This substance will absorb the oxygen gas, but not the nitrogen; hence the air in the tube will diminish in bulk, and what remains will show the proportion of nitrogen.

It was supposed by modern chemists, until lately, that oxygen was essential to combustion; and that this process was, in all cases, the combination of oxygen with the combustible body; but it has been found, that there are some other substances, as chlorine and iodine, which have also this property of supporting combustion: it is, however, the oxygen that acts in all the usual combustions in common air. The heat and light were supposed to be separated from oxygen, the base of the gas, which became fixed in the burned body. At present it is maintained by some, that combustion may be the consequence of intense chemical ac... tion, and need not depend upon any particular combination. This subject, however, remains very obscure.

Air, which has been breathed, is found to have lost its oxygen. This principle is retained in the