What has happened in this case? The carbonic acid gas from the lungs has formed with the lime in the lime water a substance called carbonate of lime, which, being insoluble in water, falls to the bottom of the glass as a white powder.
If carbonic acid gas were not present in the air blown from the lungs, this milkiness would not appear, for no other gas, except this, would alter the lime water’s clearness.
Chemistry of The Air.
Before proceeding further, it will be well to perform one or two experiments, to prove that the air we breathe is by no means the simple substance it is generally supposed to be. Although it is invisible, it must be remembered that it presses with a force of over fifteen pounds to the square inch, over the whole surface of the earth. It extends, too, to a height of some forty miles above the earth, and though it cannot be seen, it can be felt in the rush of the hurricane, and heard in the roar of the tempest. It is chiefly composed of a mixture of two gases, oxygen and nitrogen.
Did the air consist entirely of the former, people would breathe too quickly, and die in a very short time in a high fever, burned up, in fact. If only consisting of nitrogen, the human race would also die, because this element is incapable of supporting life; people would be suffocated, in fact.
Therefore, a judicious mixture of the two is essential to the life of animals. Generally, in a hundred parts of air by weight there are seventy-six parts of nitrogen to twenty-three of oxygen.
Besides these two gases, there is also a quantity of carbonic acid gas in the air, given off by all the fires and animals in the world. Of course, its amount is much greater in the great towns and manufacturing centers than in country districts.
Now herein must be recorded one of these charming arrangements which Nature has designed for the benefit of her children. Carbonic acid gas is much heavier than the air, and, therefore, sinks towards the ground, where, if allowed to accumulate, would cause the death of every animal. Fortunately, however, plants breathe in through their leaves carbonic acid gas during sunshine, and break it up into carbon and oxygen. The former, they use for building up their trunks, leaves, and flowers, while during the night they give off oxygen into the air.