Carbonic acid gas is also one of the invisible substances of the atmosphere, of which, though it forms no more than four parts in every ten thousand, yet it constitutes an important ingredient. You will understand how important it is when you are told that, from this carbonic acid in the air, all the plants which you see growing upon the land extract nearly the whole of their solid substance. When a plant dies and decays, the carbonic acid is restored to the air again. On the other hand, plants are largely eaten by animals, and help to form the framework of their bodies. Animals in breathing give out carbonic acid gas; and when they die, and their bodies decay, the same substance is again restored to the atmosphere. Hence the carbonic acid of the air is used to build up the structure both of plants and animals, and is given back again when these living things cease to live. There is a continual coming and going of this material between the air and the animal and vegetable kingdoms.
You know that though you can not see the air you can feel it when it moves. A light breeze, or a strong gale, can be just as little seen by the eye as still air; and yet we readily feel their motion. But even when the air is still it can make itself sensible in another way, viz: by its temperature. For air, like common visible things, can be warmed and cooled.
This warming and cooling of the air is well illustrated by what takes place in a dwelling-house. If you pass out of a warm room, on a winter’s day, into the open air when there is no wind, you feel a sensation of cold. Whence does this sensation come? Not from anything you can see, for your feet, though resting on the frozen ground, are protected by leather, and do not yet feel the cold. It is the air which is cold, and which encircles you on all sides, and robs you of your heat; while at the same time you are giving off or radiating heat from your skin into the air. On the other hand, if, after standing a while in the chilly winter air, you return into the room again, you feel a sensation of pleasant warmth. Here, again, the feeling does not come from any visible object, but from the invisible air which touches every part of your skin, and is thus robbed of its heat by you.