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.