The substances that are around us are commonly divided into three kinds - solids, liquids, and gases.

A solid body has a certain amount of rigidity, retains its shape and size, unless it be broken or bent. It occupies a certain space.

A liquid is a body which takes the shape of the vessel that contains it, and it occupies a certain space in the vessel.

A gas is a body, any quantity of which, however small, will fill any space, however large, within practicable limits.

A bottle full of any gas would fill a room as well as it fills the bottle. A gas will fill a space although that space is already filled with other gases, and that is what we mean when we say that one gas is to another as a vacuum. Whatever gases there are in a space, when you put another there that fills the space all the same.

This may be easily tested by taking a bottle of gas that has an offensive smell or a gas that irritates the lungs and letting it out into a room; its presence will be easily detected by every person in the room at the time.

Not merely is this true, but gases fill a space altogether irrespectively of their weights. Whether the gas is heavy or light it fills the space all the same.

If you put a gas that is heavy into a space, that gas will not sink to the bottom, but will fill the whole space.

Let us pass on to consider the gases of which atmospheric air is composed.

Air contains three gases - two in very considerable quantities, and one in small quantity.

In 10,000 parts by volume of air, 7900 are nitrogen, 2096 oxygen, and 4 carbonic acid.

These quantities vary slightly in atmospheric air in different places, and that slight variation is sufficient of itself to prove that air is not a chemical compound but a mixture.

Of these, oxygen, which is for our purpose the most important of the three, is a body that readily combines with other substances, and so it may be easily separated from the air.

If I take a small piece of phosphorus and light it in a vessel of air, the phosphorus will combine with the oxygen of that air, giving off a quantity of white fumes and depositing them in white flakes called phosphoric acid, a substance which is very soluble in water. That is another illustration of the fact that you cannot predict the properties of a chemical compound from the properties of the substances that compose it.

Oxygen will combine also with charcoal or carbon, and if carbon is burnt in oxygen it forms another substance which is found in the air, and is called carbonic acid gas. All the substances that we use for lighting and warming our rooms contain carbon, and in the process of burning in the air form carbonic acid gas. This gas is very heavy, so much so that in making it we can collect it in a bottle by the displacement of air from below, and we can pour it from one vessel into another. That is not contrary to what I said just now, that one gas let free in a space fills the whole space, because in these cases time is not allowed for it to do so.

Carbonic acid gas has among other properties the property of combining with quick lime, which is soluble in water, to produce carbonate of lime, which is nearly insoluble in water; chalk and white marble are different forms of it. If we put marble or chalk into a lime kiln the carbonic acid gas goes off and quicklime is left.

If I take a piece of marble and pour acid over it, a disengagement of carbonic acid gas takes place which may be collected in a bottle, and if lime is introduced into the bottle with it a white deposit of carbonate of lime is produced.

If a bottle of oxygen gas and a bottle of carbonic acid gas be taken, the one a light gas and the other a heavy gas, and the bottle containing the oxygen gas be inverted over the bottle of carbonic acid gas, which is the heavier, in a few minutes we shall be able to show, by means of lime-water, that some of the heavy gas has gone up to the top and mixed with the lighter gas, thus proving what I said just now, that gases in a space will mix together irrespectively of their densities.

This is an extremely important thing to remember when considering ventilation.

If you take a candle and burn it in a bottle of oxygen it burns very much more brilliantly than in air, and is consumed faster; but if you put a lighted candle into a jar of carbonic acid gas it is instantly extinguished, thus showing that oxygen gas and carbonic acid gas have very different properties.

We see, then, that in the air oxygen supports combustion, and that carbon, when burnt in the air, produces carbonic acid.

What does nitrogen do in the air?

Nitrogen is a substance which has purely negative properties: it merely dilutes the oxygen gas. It has been compared to water in a glass of brandy and water, but you must remember that the other good thing in the air is oxygen; but in brandy and water I am not at all sure that the other good thing is the brandy.

Nitrogen may be made to combine indirectly with other substances, but if I put a lighted match into it, the match will be extinguished immediately, just as if put into carbonic acid gas.