Rain-water is the purest, as we have said, because it goes through the process of distillation by nature. The sun takes it up, by evaporation, into the air, where it is condensed, and falls as rain-water. Water containing carbonate of lime will petrify or harden, as in stalactite caverns. The carbonic acid escapes from the dripping water, the carbonate in solution is deposited as a stalactite, and finally forms pillars in the cave. Sea-water contains many salts; its composition is as follows, according to Dr. Schwertzer, of Brighton:—

Water964·74372 grains.
Chloride of sodium (salt)28·05948 ”
Chloride of potassium0·76552 ”
Chloride of magnesium3·66658 ”
Bromide of magnesium0·02929 ”
Sulphate of magnesia2·29578 ”
Sulphate of lime0·40662 ”
Carbonate of lime0·03301 ”
(With traces of iodine and ammonia).
1000·00000 grains.

Fig. 349.—Stalactite Cavern.

There is much more oxygen in water than in air, as can be ascertained by analysis of these compounds. This great proportion in favour of water enables fish to breathe by passing the water through the gills. Marine animals (not fishes), like the whale,—which is a warm-blooded creature, and therefore not suited to exist without air,—are obliged to come to the surface to breathe. The density of salt water is much greater than that of fresh water, and therefore swimming and flotation is easier in the sea than in a river. We shall have more to say of water by-and-by.

NITROGEN—SYMBOL N; ATOMIC WEIGHT 14.

We have already made some reference to this gas when speaking of the atmosphere and its constituents, of which nitrogen is the principal. From its life-destroying properties it is called “azote” by French chemists, and when we wish to obtain a supply of nitrogen all we have to do is to take away the oxygen from the air by burning phosphorus on water under a glass. Nitrogen is not found frequently in solid portions of the globe. It is abundant in animals. It is without colour or smell, and can be breathed in air without danger. It is heavy and sluggish; but if we put a taper into a jar of nitrogen it will go out, and animals die in the gas for want of oxygen, as nitrogen alone cannot support life.

Fig. 350.—Obtaining nitrogen.