We cannot see air because it has no color and is perfectly transparent. If at times it appears that there is color in the air it is not the air you see, but some little particles of various substances in it. Sometimes you think when you look off toward a range of mountains or hills, for instance, that the air is blue. You know the grass and trees on the mountains are green, so it cannot be they that have turned blue, and so you think the air is blue. But it is only the sunlight reflected to your eyes from the little particles of dirt and other substances which fill the air at all times which makes the blue that you see, and not the air. Pure air is a mixture of gases without any color and is perfectly transparent. Air is nearly entirely composed of a gas called nitrogen--the remainder being oxygen with a little water and carbonic acid gas, which latter is thrown off in breathing. This is, however, but a very small percentage. Air has been and still can be reduced to a liquid state, and with the use of it in this form many seemingly wonderful things can be done, which are interesting to look at, but have not as yet become commercially practical. Why Does Thunder Always Come After the Lightning? This occurs simply because lightning or light travels so much more quickly than sound. Light travels at the rate of 186,000 miles per second, and sound travels only at the rate of 1090 feet per second when the temperature is at 32 degrees. Now, the thunder and lightning come at the same time and place in the air, but the light travels so much faster that you see the lightning often quite some seconds before you hear the thunder. In fact, you can tell quite accurately how far away from you the flash of lightning and clap of thunder are by taking a watch and noting the number of seconds which elapse between the flash of the lightning and the time when you hear the roll of the thunder. If as much as five seconds elapse you can figure that it was about a mile away from you, since sound travels only about 1100 feet per second and there are 5280 feet in a mile. When the thunder and lightning come close together you may know that it is near by, and when they come at the same time you may be sure it is very close. When, therefore, you see the lightning and then have to wait several seconds for the noise of the thunder, you may rest easy about the lightning hurting you, because you know then it is too far away to harm you, and when it is so close that the lightning and thunder come simultaneously, there is no use being afraid, because if you were to be struck you would have been struck at the same instant or before you would have had time to notice that the lightning and thunder come together. How Big Is the Sun?