Before proceeding further, it will be well to perform one or two experiments, to prove that the air we breathe is by no means the simple substance it is generally supposed to be. Although it is invisible, it must be remembered that it presses with a force of over fifteen pounds to the square inch, over the whole surface of the earth. It extends, too, to a height of some forty miles above the earth, and though it cannot be seen, it can be felt in the rush of the hurricane, and heard in the roar of the tempest. It is chiefly composed of a mixture of two gases, oxygen and nitrogen.

Did the air consist entirely of the former, people would breathe too quickly, and die in a very short time in a high fever, burned up, in fact. If only consisting of nitrogen, the human race would also die, because this element is incapable of supporting life; people would be suffocated, in fact.

Therefore, a judicious mixture of the two is essential to the life of animals. Generally, in a hundred parts of air by weight there are seventy-six parts of nitrogen to twenty-three of oxygen.

Besides these two gases, there is also a quantity of carbonic acid gas in the air, given off by all the fires and animals in the world. Of course, its amount is much greater in the great towns and manufacturing centers than in country districts.

Now herein must be recorded one of these charming arrangements which Nature has designed for the benefit of her children. Carbonic acid gas is much heavier than the air, and, therefore, sinks towards the ground, where, if allowed to accumulate, would cause the death of every animal. Fortunately, however, plants breathe in through their leaves carbonic acid gas during sunshine, and break it up into carbon and oxygen. The former, they use for building up their trunks, leaves, and flowers, while during the night they give off oxygen into the air.

This is the reason why plants and trees planted in the streets so largely help to sweeten and purify the foul air of a great city.

An experiment to prove that the atmosphere does consist of nitrogen and oxygen, may be prettily proved in the following simple manner: A glass marmalade jar, or a soup-plate filled with water, and a piece of phosphorus as large as a pea, are the only things necessary. Take very great care not to touch the phosphorus, for the heat of the hand is sufficient to set it on fire, and a terrible wound would be caused.

Place the phosphorus in a match-box on the surface of the water, touch it with a lighted match, and put the jar-mouth downwards over it to the bottom of the plate. The phosphorus burns with a dazzling brilliancy, and gives off dense white fumes. At the same time the water rises a third of the way up the jar, but not to the top, thus showing that all the invisible matter has not been consumed. The white soon settles into the water and is dissolved. The phosphorus has combined with the oxygen in the jar and forms phosphoric oxide, which dissolves in water. There is then only the nitrogen left. The disappearance of the oxygen allows the water to fill up the space it formerly occupied.

This may be followed by another experiment.

To show that oxygen is necessary for the support of combustion, fix two or three pieces of wax taper on flat pieces of cork, and set them floating on water in a soup-plate, light them, and invert over them a glass jar.

As they burn, the heat produced may perhaps at first expand the air, so as to force a small quantity out of the jar, but the water will soon rise in the jar, and continue to do so until the tapers expire, when you will find that a considerable portion of the air has disappeared, and what remains will no longer support flame.

The oxygen has been converted partly into water, and partly into carbonic acid gas, by uniting with the carbon and hydrogen of which the taper consists, and the remaining air is principally nitrogen, with some carbonic acid. The presence of the latter may be proved by decanting some of the remaining air into a bottle, and then shaking some lime water with it, which will absorb the carbonic acid and form chalk.

Into an ale glass, two thirds full of water at about 140 degrees, drop one or two pieces of phosphorus about the size of peas, and they will remain unaltered. Then take a bladder containing oxygen gas, to which is attached a stop cock and a long fine tube. Pass the end of the tube to the bottom of the water, turn the stop cock, and press the bladder gently. As the gas reaches the phosphorus it will take fire, and burn under the water with a brilliant flame, filling the glass with brilliant flashes of light dashing through the water.

Into another glass put some cold water; introduce carefully some of the salt called chlorate of potash; upon that drop a piece of phosphorus; then let some strong sulphuric acid (oil of vitriol) trickle slowly down the side of the glass, or introduce it by means of a dropping bottle.

As soon as the acid touches the salt the latter is decomposed, and liberates a gas which ignites the phosphorus, producing much the same appearance as in the last experiment.

Into the half of a broken phial put some chlorate of potash, and pour in some oil of vitriol. The phial will soon be filled with a heavy gas of a deep yellow color. Tie a small test tube at right angles to the end of a stick not less than a yard long, put a little ether into the tube, and pour it gently into the phial of gas, when an instantaneous explosion will take place, and the ether will be set on fire. This experiment should be performed in a place where there are no articles of furniture to be damaged, as the ingredients are often scattered by the explosion, and the oil of vitriol destroys all animal and vegetable substances.

Into a jar containing oxygen gas introduce a coil of soft iron wire, suspended to a cork that fits the neck of the jar and having attached a small piece of charcoal to the lower part of the wire, ignite the charcoal. The iron will take fire and burn with a brilliant light, throwing out bright scintillations, which are oxide of iron, formed by the union of the gas with the iron; and they are so intensely hot that some of them will probably melt their way into the sides of the jar, if not through them.

But by far the most intense heat, and most brilliant light, may be produced by introducing a piece of phosphorus into a jar of oxygen. The phosphorus may be placed in a small copper cup, with a long handle of thick wire passing through a hole in a cork that fits the jar. The phosphorus must first be ignited; and as soon as it is introduced into the oxygen, it gives out a light so brilliant that no eye can bear it, and the whole jar appears filled with an intensely luminous atmosphere. It is well to dilute the oxygen with about one-fourth part of common air, to moderate the intense heat, which is nearly certain to break the jar if pure oxygen is used.

The following experiment shows the production of heat by chemical action alone: Bruise some fresh-prepared crystals of nitrate of copper, spread them over a piece of tin foil, sprinkle them with a little water; then fold up the foil tightly, as rapidly as possible, and in a minute or two it will become red hot, the tin apparently burning away. This heat is produced by the energetic action of the tin on the nitrate of copper, taking away its oxygen in order to unite with the nitric acid, for which, as well as for the oxygen the tin has a much greater affinity than the copper has.