“If the object of Rome had been civilization, instead of conquest and plunder, the development of the Germans might have commenced much earlier and produced very different results.”—_Taylor._
When we begin to look attentively at the world around us, one of the first things to set us thinking is the air. We do not see it, and yet it is present wherever we may go. What is this air?
Although invisible, it is yet a real, material substance. When you swing your arm rapidly up and down you feel the air offering a resistance to the hand. The air is something which you can feel, though you can not see it. You breathe it every moment. You can not get away from it, for it completely surrounds the earth. To this outer envelope of air, the name of _atmosphere_ is given.
The air is not a simple substance, but a mixture of two invisible gases, called nitrogen and oxygen. But besides these chief ingredients, it contains also small quantities of other substances; some of which are visible, others invisible. If you close the shutters of a room, and let the sunlight stream through only one chink or hole into the room, you see some of the visible particles of the air. Hundreds of little motes, or specks of dust, cross the beam of light which makes them visible against the surrounding darkness, though they disappear in full daylight. But it is the invisible parts of the air which are of chief importance; and among them there are two which you must especially remember—the vapor of water and carbonic acid gas. You will soon come to see why it is needful for you to distinguish these.
Now what is this vapor of water? You will understand its nature if you watch what takes place when a kettle boils. From the mouth of the spout a stream of white cloud comes out into the air. It is in continual motion; its outer parts somehow or other disappear, but as fast as they do so they are supplied by fresh materials from the kettle. The water in the kettle is all the while growing less, until at last, if you do not replenish it, the whole will be boiled away, and the kettle left quite dry. What has become of all the water? You have changed it into vapor. It is not destroyed or lost in any way, it has only passed from one state into another, from the liquid into the gaseous form, and is now dissolved in the air.