The air which we breathe is rarely quite pure, and is often very impure. This is especially the case in city life and within our houses. Pure air is only to be found in the open country, the mountains, or at the sea-side. In addition to its normal oxygen and nitrogen, many other gases make their way into it, some of them, being very unhealthful. There are also solid particles of "dust," of a great variety of materials, animal, vegetable, or mineral, many of them more or less harmful. The worst of them are the floating bacteria, living germs of disease, which inhabit air and water alike; the great majority of these are harmless, some of them are deadly in their effects

Of the impure gases in the air, some of the worst are of our own production. We are constantly breathing out matter which is poisonous to the system if breathed in again. This is largely carbon dioxide (or carbonic acid gas), with small quantities of organic poisons, the waste of the system.

We can easily understand how it is that pure air becomes poisoned by respiration, the specially dangerous products being the carbon dioxide and the organic matters. The total amount of carbon dioxide breathed out in an hour is about 6 cubic feet. While this is an injurious gas, it is probable that the bad effects of breathing respired air are more due to the poisonous organic matter, as it is found that while an artificial atmosphere containing 1 part of carbon dioxide.

Misplaced Pipe

In 100 of air causes but little discomfort when breathed, yet if an already respired air containing only 1 part of carbon dioxide in 1,000 of air is breathed much discomfort is experienced. This organic poison is probably composed partly of an organic vapor from the lungs, and partly of solid matter from the lining of the mouth and air passages. It is difficult to find out the exact quantity of organic matter present, but it varies exactly in proportion to the quantity of carbon dioxide, and the amount of this in respired air is therefore taken as the standard of impurity.

The Air from Sewage and Sewers.

This is found to contain a great diminution of the oxygen, a large increase of the carbon dioxide, and many other gases, such as sulphureted hydrogen, sulphide of ammonium, marsh-gas, etc. A more harmful constituent is found in the numerous germs present, which are probably thrown into the air of the sewer by the bursting of bubbles on the surface of the putrefying sewage.

The air from churchyards contains carbon dioxide in excessive amount, various vapors of ammonia, offensive and putrid gases, and many germs.

Air polluted by Trades.

These impurities depend, of course, on the nature of the trade. We may have hydrochloric acid, sulphur dioxide, sulphurous acid, ammonia, and sulphureted hydrogen from chemical

works; carbon dioxide and monoxide and sulphureted hydrogen from brickfields; nauseous organic vapors from glue refining, bone burning, fat boiling, candle making, and slaughter houses; and various vegetable and mineral impurities from near works where cotton, linen, flint, or iron particles are thrown into the atmosphere. Nor must we forget the air of workrooms polluted by various products of manufacture, such as lead, phosphorous, flax, etc.

The air of towns must necessarily be very impure, owing to the presence of the injurious products given off by combustion, respiration, sewers, and trades; we find a lessened amount of oxygen, an increased amount of carbon dioxide, and a fairly large amount of solid matter, both inorganic and organic. It is also found that it is especially in the narrow streets of crowded parts of the town that the atmosphere is particularly foul. In the open spaces and wide streets the impurities are not nearly so great.

In close rooms the air is made impure by products of combustion (as from the burning of gas) and by respiration; the impurities thus caused may be very great, even to the extent of 3 parts of carbon dioxide in 1,000 of air. In a room in Leicester, containing six persons, with only 51 cubic feet of air space each, and with three gaslights burning, the amount of carbon dioxide was found to be over 5 parts per 1,000 of air.