Air contains a small quantity of a substance we call ammonia; a very small quantity only, amounting to about 4 parts in 10,000,000 parts of air.

It contains also, under ordinary circumstances, a very small quantity of a substance not yet mentioned-viz., ozone.

The presence of ozone in the air has been doubted by a good many people ; but all doubts have now been set at rest by a very admirable series of experiments. It is another form of oxygen gas, much more powerful than ordinary oxygen. It has the power of combining with most substances under certain conditions, and of combining very readily with the substances of which foul organic matters are composed, and so purifying the air.

One of the ways of preparing ozone is to take a few sticks of phosphorus, a substance which readily combines with oxygen, and to leave them for a time exposed in moist air. If you take a bottle, and put some water into it, and in the water some sticks of phosphorus projecting out of the water in the bottle, and let it stand, part of the oxygen in the air in the bottle will be turned into ozone. This may be tested thus:-

Ozone is capable of liberating a substance called iodine, from some of its compounds, and iodine will strike a beautiful blue colour with starch; therefore, if you put a strip of paper that has been dipped in a mixture of a solution of iodide of potassium and starch-paste, into the air that has been exposed to the influence of phosphorus, if there is any ozone in the air the paper will be turned blue.

Ozone is present in very small quantities in pure air, in the air that blows from the sea, after a thunderstorm, and during a fall of snow. Its occurrence in the air is accounted for in another way. Whereas animals take oxygen out of the air, and put carbonic acid gas into it, the green parts of plants, under the influence of sunlight, do the reverse, they take the carbonic acid gas out of the air, and give back oxygen gas in exchange during the daytime.

The flowers of plants, on the contrary, and fruits while ripening, are all like animals in this respect; and so it is not right to have flowering plants in bedrooms.

It has been shown that the green parts of plants give out a certain quantity of oxygen in the form of ozone. There is always a considerable quantity of ozone present where there is much growing vegetation, and this is especially the case where there is vegetation of certain kinds.

I daresay you have heard the statement that ozone is connected with the presence or absence of epidemic diseases, and that it is never present when cholera is prevalent. A sufficiently large number of experiments have shown, however, that it is as often present as absent during the prevalence of epidemics.

If you examine the air in a crowded hospital ward you will not find ozone there, because any that gets in with the fresh air from without is immediately used up in oxidising the foul organic matters present in the air. You cannot prepare ozone by the ordinary methods in foul air, because as fast as it is produced it is used up by the impurities; it is, in fact, nature's disinfectant.

Ozone is an exceedingly irritating substance. We could not breathe air containing much of it because it irritates the respiratory organs, and this is why it has been said to favour the prevalence of influenza and bronchitis.

There is also always a certain amount of water dissolved in the air, the amount of which varies at different temperatures. There may, too, be a large amount of water suspended in the air in the form of mist or fog.

Besides these matters, which are ordinarily present in the air, there is also a variable amount of solid matters suspended in the air. Dust, consisting of sea salt, which is blown up and carried, immense distances; containing also a large quantity of particles derived from the earth, and fine sand. Ships sailing 600 or 800 miles from land sometimes have their sails covered with fine sand blown from the great African desert, and eruptions of volcanoes often charge the air with solid particles, which travel astonishing distances.

We see from this that there is a considerable amount of suspended mineral matter in the air ; there is also a variable amount of organic, matter, both living and dead.

Dead organic matter, from the exhalations of animals, and from the decomposition of animal and vegetable substances. There is a very small amount, indeed, of these things in pure air, because the oxygen and the small quantity of ozone in the air decompose them, and turn them into other substances which are harmless.

There are living substances in the air, both living animals and vegetables, many exceedingly small, and only to be seen by the aid of the microscope.

These suspended matters may contain, and, no doubt, sometimes do contain, the poisons of infectious diseases, which are solid particles suspended in the air around persons infected, and from them, doubtless, these particles get into the air.

We will now consider other sources from whence carbonic acid gets into the air.

There are natural sources, besides the respiration of animals, of carbonic acid-viz., volcanoes, and mineral springs in volcanic countries, which very often emit large quantities of this gas.

It is also given out in various manufacturing processes, in the making of beer both during the fermentation of the grains to make malt, and during the subsequent fermentation, when the sugar and water is transformed into alcohol and carbonic acid; and instances are on record where persons have been suffocated by going down into the brewers' vats.

Carbonic acid gas is in itself poisonous. It is not like nitrogen, a mere harmless substance, which poisons you when you go into it because there is no free oxygen, but an animal will die in an atmosphere containing plenty of free oxygen, if it contains over 10 or 12 per cent of carbonic acid gas.