Experiment 21.-Put a small iron nail into a test tube, and drench it with twenty drops of common sulphuric acid; it is not acted upon. But if you add a little water, about four or five times more than the acid, a brisk effervescence will ensue, and the iron will be dissolved. The strong acid may be heated to boiling in iron vessels without acting upon them, which is by no means the case with the diluted acid. In this experiment, hydrogen escapes, and iron sulphate remains in solution. Zinc behaves in a similar manner. Consequently, the diluted acid must be employed for dissolving such metals. But there are also metals which dissolve only in the stronger acid, with the aid of heat; for instance, copper, silver, &c. This has been treated of under sulphurous acid.

SELENIUM, Se = 79.5, AND TELLURIUM, Te = 129.

These elements are very scarce and unimportant; they occur combined with certain metals in some rare minerals. They both resemble sulphur in their properties, but partake more of the character of metals in appearance. Their compounds with hydrogen and oxygen correspond perfectly to those of sulphur.

CHAPTER III.

NON-METALLIC TRIADS.

(Nitrogen, Phosphorus, Boron.)

NITROGEN.

Symbol, N = 14. Formula, N.

In the free state as gas nitrogen constitutes nearly four-fifths of the atmosphere, the remaining one-fifth being oxygen. In a state of combination it is an important constituent of organic structures. It is also found in the mineral kingdom, as in saltpetre or nitre, a salt occurring as an incrustation on the earth in India and elsewhere; from this salt the name nitrogen (generator of nitre) is derived. The atomic weight of nitrogen is 14, and the molecule consists of two atoms. Hence the specific gravity of the gas is 14 (p. 72).

The most convenient and simple method for the preparation of nitrogen consists in removing the oxygen from a confined portion of air by the ordinary process of com

bustion.

Fig. 58.

Experiment 1.-Fasten a piece of sponge to a wire, drop some alcohol upon it, and hold the wire in a vessel containing water so that the sponge may be some inches above the water. Then kindle the spirit, and immediately place an empty bottle over it, so that the mouth of it may dip into the water; the flame will soon cease burning, and some of the water will rise into the bottle, in proportion to the amount of air disappearing during the combustion. The consumed air was oxygen, which united with the constituents of the alcohol. Close the bottle tightly

with the finger, shake it briskly, and again open it below the water, when a little more water will enter. The air which remains in the bottle is nitrogen; it is sometimes called azote (a, privative, and (wń, life), from its inability to support respiration.

Other combustibles may be used for the purpose of abstracting the oxygen from air, such as sulphur, charcoal, &c., but phosphorus is the best; for the products of combustion of the other substances, being gaseous, are not easily got rid of, and consequently contaminate the nitrogen, whereas the phosphorus compound produced is solid, and moreover very soluble in water. Moreover phosphorus burns till all the oxygen is removed, which is not the case with many combustibles.

Experiment 2.-Place a small piece of dry phosphorus in a little basin floating in water. Ignite the phosphorus and place over it a bell-jar or the mouth of a wide-necked bottle (Fig. 58). The phosphorus will burn until the whole of the oxygen is consumed. Dense white clouds of phosphoric anhydride (PO) are formed and fill the jar, but these after a time are precipitated in snowy flakes, and are rapidly dissolved by the water. The water will rise in the bottle, partly from the removal of oxygen and partly from some air having been expelled at the beginning of the experiment, in consequence of the expansion produced by the heat (p. 34). When the white fumes have entirely disappeared the bottle may be shaken so as to cause the basin to fill and sink, and the nitrogen transferred in the pneumatic trough to a series of smaller bottles.

Nitrogen is incombustible, except at a very high temperature. It will not support combustion, for a lighted taper immersed in a jar of it is extinguished. It has neither colour, smell, nor taste, and in a chemical point of view it must be regarded as a very inert body, since it does not combine directly with any other substance except with great difficulty.

Nitrogen gas is erroneously called azote, as we are continually breathing it without perceiving any injurious effects from it; it stops respiration only when it contains no oxygen, and because it contains none. The human body is so constructed, that it will not thrive on substances intended

as nourishment if they are presented to it in their purest form. Strong alcohol acts as a poison, but when diluted with four or five times its quantity of water, as in wine, it is invigorating. Even the respiration of oxygen would soon destroy life, were it not diluted with four times its volume of nitrogen, as in atmospheric air.

Besides oxygen and nitrogen, air contains vapour of water and carbonic anhydride. The presence of the former is rendered obvious by the fall of rain, snow, dew, &c.,; and that of carbonic anhydride can easily be shown by letting limewater remain exposed to the air, or by shaking it in a flask containing air. This occasions a cloudiness in the liquid. If you ask, What is the source of this carbonic anhydride? the reply is, It is formed wherever substances are burning, wherever men and animals are breathing, and wherever decay and putrefaction are taking place. (Page 126.) The following is a close approximation to the average composition of the air:

The carbonic anhydride amounts in pure country air to about three volumes in 10,000. In towns it is more abundant.

The air is not a compound, but only a mixture. It varies slightly in composition, which compounds never do (page 63), and its properties are intermediate between those of its constituents, whereas those of a compound are entirely different.

In crowded rooms, and other confined places, the air becomes deteriorated; that is, poorer in oxygen and richer in carbonic anhydride.

That the air also contains other foreign ingredients is not strange, since it is the constant receptacle of volatile substances and dust. The air coming from the Spice Islands, even at the distance of eight or ten miles, is impregnated with the odour of cinnamon and cloves. The dust contained in the air can be discerned in the sunbeam, &c. Minute

quantities of ammonia and nitric acid are also found in the air. The presence of ozone has already been noticed. The specific gravity of dry air, at standard pressure and temperature, is 14·47, that is, it is about 14 times heavier than hydrogen.

AMMONIA, NH ̧.

This important gas can be prepared in minute quantity by the direct union of its elements. It is also frequently. formed during the putrefaction of organic bodies, but its most convenient source is in the “ammoniacal liquor" of gas-works. From this a white substance called sal-ammoniac -the ammonium chloride of chemists-is manufactured.

Experiment 1.—Mix about half an ounce of powdered salammoniac with about one ounce of quick lime. A pungent odour of hartshorn is perceived. Introduce the mixture into a dry Florence flask fitted with a cork and a short tube which communicates by an india-rubber tube with a long straight tube. Apply a gentle heat, and ammonia gas will come off in abundance. It is very soluble in water, but being lighter than air it may be collected by upward displacement, that is, by holding a bottle mouth downwards over the tube and allowing the light gas to displace the heavier air. The reaction by which the gas is prepared is as follows: :

2NH,C1+CaO = 2NH + CaCl+H,O.

Calcium chloride remains in the flask, together with the excess of lime.

Experiment 2.-Plunge a taper into a bottle of ammonia held with its mouth downwards. The taper will be extinguished and will not ignite the gas.

Experiment 3.-We must not too hastily conclude from the preceding experiment that ammonia cannot be made to burn. Hold the end of the tube from which the gas is issuing in a flame. You will perceive that the gas burns with a greenish flame as long as the jet is heated, but that it goes out when the lamp is removed. Ammonia will burn when strongly heated. During its combustion its hydrogen is oxidized to water, and its nitrogen escapes free.