graduated tube standing over water, a piece of phosphorus. The known quantity of air can be determined by observing the water point in the graduated tube. The arrangement should now be left for twenty-four hours. The whole of the oxygen will be absorbed by slow combination with the phosphorus. The remaining gas may now be read off, which is nitrogen. Another plan is to mix air with hydrogen, and explode by an electric spark; the hydrogen and oxygen combine, and the remaining volume of gas is read off.

Nitrogen is not combustible, nor is it a supporter of combustion or respiration; it is destitute of any acid or alkaline reaction. It is estimated that six tons of air pass through a blast furnace every hour. While the oxygen is most active in forming new compounds the nitrogen escapes unchanged. Its chief function is to modify the violent action of the oxygen, and render the atmosphere suitable to the wants of life.

There are five compounds of oxygen and nitrogen, which may be taken as a good illustration of the law of multiple proportions in chemistry :

The protoxide of nitrogen NO, is sometimes called laughing-gas, from the peculiar exhilarating effects produced when inspired. It is prepared by heating moderately in a retort the nitrate of ammonia. Four ounces of salt produce one cubic foot of gas. It should be collected over warm water, and care should be taken that the salt is pure. The change which takes place is illustrated by the following equation

Nitrous oxide is a supporter of combustion. The inhalation of this gas is sometimes attended with serious results.

The binoxide of nitrogen NO2, may be prepared by introducing into the hydrogen apparatus some copper turnings, a little water, and nitric acid, added by the funnel. The gas may be collected over water :—

3Cu + 4NO, 3(CuO,NO5) + NO2.

=

(3Cu + 4H,NO, = 3Cu,NO, + 4HO + NO,).

The gas obtained in this manner is colourless and transparent; but in contact with air or oxygen it produces deep red fumes, which are sufficient to distinguish it from all other gases. This may be beautifully illustrated by passing some binoxide of nitrogen into a gas jar containing oxygen, standing over the pneumatic trough, or allowing the fumes to pass into the atmosphere.

Nitrous acid, NO,.-When four volumes of dry binoxide of nitrogen are mixed with one volume of dry oxygen, and exposed to as low a temperature as can be obtained by freezing mixtures, a thin mobile liquid of a green colour is produced. This is nitrous acid, NO3.

Hyponitric acid, NO,.-If nitrate of lead be heated in a retort of hard glass it is decomposed into the protoxide of lead, oxygen, and hyponitric acid. It may be collected in a receiver surrounded with a freezing mixture.

Great obscurity hangs over these two compounds of nitrogen and oxygen. The red fumes produced when the binoxide of nitrogen comes in contact with oxygen are chiefly due to the formation of hyponitric acid. Other oxygen compounds are also formed.

Nitric acid, HO,NO, (H,NO,).-Known as aquafortis, and one of the most important compounds of nitrogen. In hot dry climates, the soil is sometimes covered with a white, downy-looking salt. When this salt is collected, dissolved, filtered, and crystallized, it produces the nitre or

saltpetre of commerce: it is a compound of potash and nitric acid. To obtain nitric acid from this salt, mix equal weights of sulphuric acid and nitrate of potash in a glass retort, cover the receiver, which may be a Florence flask, with a wet cloth, and apply heat. Red fumes at first rise

H

Fig. 12.

in the retort, and pass to the receiver; these fumes gradually disappear, and a pale yellow liquid collects in the receiver, which is nitric acid. Decomposition :

KO,NO, HO,SO, KO,SO, + HO,NO,.

=

(K,NO,H,SO, K,SO, + H,NO.).

=

On a large scale earthen retorts and condensing vessels are used, and nitrate of soda substituted for nitrate of potash.

Pure nitric acid is a colourless liquid. It fumes when exposed to air. It is partially decomposed by light. It is intensely acid, and reddens vegetable blues. It stains the skin and nails yellow, and is used for producing yellow colours on silken fabrics. It is also used for assaying, and as a solvent for tin in calico printing. It is a powerful oxidising agent, attacking most of the metals with great

violence.

The anhydrous acid (anhydride) NO,, is a volatile

solid, crystallizing in brilliant colourless prisms. It decomposes at a temperature a little above that of boiling water; yielding hyponitric acid and oxygen. It does not, however, possess acid properties till it is dissolved in water, when it forms ordinary nitric acid.

Nitric acid forms a series of salts, all of which are soluble in water. After a thunderstorm, nitric acid, in small quantities, has been detected in rain water. It is supposed by some, to be produced in the air by electricity.

When nitrates are heated with an excess of hydrochloric acid they are converted into chlorides, with the formation of hyponitric acid and the disengagement of free chlorine.

KO,NO, + 2HCl = KCl + 2HO + NO, + Cl.

When paper is soaked in a solution of potash, and dipped in nitric acid it becomes touch-paper. Cotton, soaked in nitric acid, washed and dried at a temperature not exceeding 120°, forms gun-cotton. The low temperature at which it explodes has prevented its substitution for gunpowder.

PHOSPHORUS.

Symbol, P; Combining Equivalent, 32.

This substance was discovered by the alchemists two hundred and fifty years ago. At ordinary temperatures it is a solid, waxy-looking substance, easily cut with a knife. It is poisonous. In the dark it shines with a faint blue light, and is highly combustible, often taking fire with the slightest touch or friction. Phosphorus is always kept under water, and great care is required in experiments with it.

Phosphorus is prepared from bones; they are first well calcined, reduced to powder, and mixed with two thirds their weight of dilute sulphuric acid; after some time the

mixture is filtered. The sulphate of lime which has been formed by the sulphuric acid is a very insoluble compound and easily separated on the filter. The liquor is then evaporated to the consistence of treacle, mixed with charcoal powder, and transferred to an iron vessel which is raised to a high temperature till all moisture is expelled. After this mixture has been thoroughly dried it is heated in a stoneware retort, from which a large tube dips into water contained in the receiver. The phosphorus distils over in a soft condition, and is moulded in hot water into cylindrical sticks. The change which takes place in the retort is easily understood. Phosphoric acid (anhydrous) contains one equivalent of phosphorus and five equivalents of oxygen. The oxygen combines with five equivalents of carbon, forming carbonic oxide, which escapes :

PO, + 5C = P + 500.

The phosphates play a very important part in the solid structure of animals, giving stiffness and rigidity to the bones. The dried skeleton of a man weighs about 10 or 12 lbs., and contains about 2 lbs. of phosphorus. Traces of phosphorus have been found in the brains, flesh, and blood of all animals. There is an important modification of phosphorus discovered by Professor Schrotter, called allotropic phosphorus, in which state the active properties of ordinary phosphorus are suspended; it is of a brickred colour, insoluble in bisulphide of carbon, does not become luminous in the dark, has no tendency to combine with oxygen, is not poisonous, and 'may be carried about without fear.

Phosphorus is largely used for the manufacture of lucifer matches. As the phosphorus would be likely to ignite by exposure to the atmosphere, it is mixed up with glue or gum water. The ends of the matches are first dipped in sulphur, then into the phosphorus solution, and afterwards into a thick mixture of chlorate of potash or nitre, or some other