1160. Atmospheric air consists principally of nitrogen and oxygen, but it also contains a portion of carbonic acid gas, which is produced by the breathing of men and other animals, &c. It is well known that atmospheric air is essential to the support of animal life, and that the lungs of animals require a constant supply of fresh oxygen, and reject the carbonic acid it contains; which, if they are forced again to inspire it, is destructive to life. Thus, if a human being is shut up in a confined space, where he can obtain no fresh air, but is forced to breathe again the air which has already passed through his lungs, and which he has already carbonated, he will soon die of suffocation. In the like manner, people are killed by the fumes of charcoal, which soon generates a sufficient quantity of carbonic acid gas to destroy life. It is a beautiful example of the economy of nature, that the same gas which is injurious to animals is useful to plants, and that plants by absorbing the carbonic acid gas which they find in the atmosphere, purify it, and render it wholesome for human beings.
