It is not my purpose to go into a learned, scientific account in this place, or even in this work, of the constitution of the atmosphere. A few plain statements are all that are indispensable. The atmosphere which we breathe is composed of two different airs or gases. One of these is called oxygen, [Footnote: Oxygen gas is the chief supporter of combustion, as well as of respiration. It is the vital part, as it were, of the air. No animal or vegetable could long exist without it. And yet if alone, unmixed, it is too pure and too refined for animals to breathe. Nitrogen gas, on the contrary, while alone, will not support either respiration or combustion; mixed, however, with oxygen, it dilutes it, and in the most happy manner fits it for reception into the lungs.] and the other nitrogen. There is another gas usually found with these two, in smaller quantity, called carbonic acid gas; but whether it is necessary, in a very small quantity, to health, chemists, I believe, are not agreed. One thing, however, is certain--that if any portion of it is healthful, it must be very little--not more, certainly, than one-fiftieth or one-hundredth of the whole mass. It is by means of the oxygen it contains, that air sustains life and combustion. Were it not for this, neither fires nor candles would burn, and no animal could breathe a single moment. Breathing consumes this oxygen of the air very rapidly. When the oxygen is present in about a certain proportion, combustion and respiration go on well, but when its natural proportion is diminished, the fire does not burn so well, neither does the candle; and no one can breathe so freely. Not only are breathing and combustion impeded or disturbed by the diminution of oxygen in the atmosphere, but just in proportion as oxygen is diminished by these two processes, or either of them, carbonic acid is formed, which is not only bad for combustion, but much worse for health. If any considerable quantity of it is inhaled, it appears to be an absolute poison to the human system; and if in _very large quantity_, will often cause immediate death. It is this gas, accumulated in large quantities, that destroys so many people in close rooms, where there is no chimney, nor any other place for the bad air to escape. But it not only kills people outright--it partly kills, that is, it poisons, more or less, hundreds of thousands.