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.

Now, just as a lighted candle lives in oxygen gas, and lives in air because there is oxygen gas in the air, which combines with the carbon in the candle when it is lighted, so our life goes on in precisely the same way, and for precisely the same reasons. You know from my lecture on respiration, that when we breathe we take in air into our lungs, oxygen gas gets into the blood, and carbonic acid gas comes out into the air.

So, then, the substance that it is important for us to have in sufficient quantities is oxygen gas, and just as the candle goes out in nitrogen gas and carbonic acid gas, so animals cannot live in either of those gases, and just for the same reason. There is, however, this difference-an animal cannot live in nitrogen gas, simply because of the absence of free oxygen ; but carbonic acid gas kills an animal put into it, because it is a poisonous gas, and, if in sufficient quantity, it will kill an animal although plenty of free oxygen be present.

When we breathe we take oxygen out of the air and put carbonic acid into its place, and, besides this, we add to the air organic matter and moisture. This decomposing organic matter that we add to the air is a thing of far greater importance than either the decrease of oxygen or the increase of carbonic acid gas. In air that has been breathed, the most deleterious thing is the foul organic matter.

The second thing that is of importance is the diminution of oxygen, and the last thing that is of importance is the increase of carbonic acid gas.

One reason why I put the presence of carbonic acid as the last consideration is this, that animals will live for a long time in air that contains far more carbonic acid than the air we breathe out, while the foul organic matter, to the same extent, would be most prejudicial to them.

The carbonic acid gas in the air that we breathe out can easily be shown by taking a solution of lime water, and inserting in it a glass tube, and breathing through the tube into the lime water, when it will be found, that, while clear before blowing into it, afterwards it becomes gradually like milk ; this is caused by the action of the carbonic acid gas upon the lime, forming carbonate of lima.

What are the results of breathing air that contains these impurities ?

Overcrowding in an extremely excessive form often results in death from a disease that kills very fast, a kind of putrid fever.

The most famous instance of severe overcrowding is that of the Black Hole of Calcutta, where 146 persons were placed in a space which was 18 feet each way, with two small windows on one side. , They were placed in that hole at eight o'clock in the evening, and at six o'clock the next morning 123 were found dead, and only twenty-three survived, and these all suffered from a putrid fever accompanied with eruptions of boils.

Typhus fever finds its home in overcrowded dwellings. l?ut what is even more important is that those who are confined to small overcrowded ill-ventilated rooms suffer from consumption, the great plague of our climate, and so prevalent is consumption among people who live in overcrowded rooms, without a sufficient amount of air to breathe, that one author has stated his deliberate conviction that the essential cause of consumption is breathing air that has been breathed before.

How much air, then, does an individual require to breathe ? Well, it can be found by direct calculation from the carbonic acid he gives out, or from the diminution of oxygen in the air that he breathes ; it has also been found, from the state of the air in different places, that the air of a room is not fresh, but must be described as stuffy if the carbonic acid exceeds 6 parts in 10,000 of air (that in the outer air being four parts). You must bear in mind that it is not by reason of the small increase of carbonic acid that the air is impure, but the amount of carbonic acid is a convenient test of the total respiratory impurity.

It has been found, from these considerations, that each individual requires 3000 cubic feet of air per hour, so that if he is in a space of 1000 cubic feet, that air must be changed three times per hour.

There are other diseases besides those already mentioned, that are favoured in various degrees by breathing air that has been breathed before, but as consumption is of the greatest importance to us, I think it sufficient to insist upon the fact, that if you live in an overcrowded place and breathe air that has been breathed before, you are more liable to suffer from consumption, not to say anything of other diseases, than you would be if you lived in a purer atmosphere.

It is not difficult to find out if the air contains too much carbonic acid.

I showed you that carbonic acid gas forms a white deposit in lime water.

Now, if you mix a certain volume of air with a known quantity of lime water, shaking them up together, it is possible, by finding "out how much of the lime is not deposited, to tell how much has combined with the carbonic acid, and so how much carbonic acid there was in the air: That is somewhat a complicated proceeding, but it can be modified by a very simple method, which Dr. Angus Smith calls the " household method," of examining air for carbonic acid.

lime water can be easily made by taking a little quicklime and putting it into water in a basin. You must not put the quicklime into a bottle and then pour water upon it, else it will break the bottle, as slaking lime gives out much heat. Let it stand, and the water will dissolve a certain quantity of the lime, the excess falling to the bottom and leaving a clear solution of lime, which you can pour off and keep in a stoppered bottle. If you put half an ounce of this clear lime water into a clean and dry 10£ ounce wide-mouthed stoppered bottle with the air in it, and then shake it up, if the lime water becomes turbid then you know the air in that room contains more than six parts of carbonic acid gas in 10,000, that is to say, that the room is not supplied with sufficient air, because you will remember I told you that the carbonic acid in the air of a room should not exceed that proportion.