On a summer morning the sky is often free from cloud. As the day advances, and the earth gets warmed, more vapor is raised; and as this vapor, borne upward by the ascending air-currents, reaches the higher and colder parts of the atmosphere, it is chilled into the white fleecy clouds which you see forming about mid-day and in the afternoon. Toward evening, when less evaporation takes place, the clouds cease to grow, and gradually lessen in size until at night the sky is quite clear. They have been dissolved again by descending and coming in contact with the warm air nearest to the earth. Again, you have often noticed that clouds move across the sky. They are driven along by upper currents of air, and of course the stronger these currents are the faster do the clouds travel. In this way the sky is sometimes completely overcast with clouds which have come from a distance. You are well aware that rain always comes from clouds in the sky. When the sky is clear overhead, no rain falls. Only when it gets overcast does the rain come. You can watch a dark rain-cloud gather itself together and discharge a heavy shower upon the earth. When a cold glass is brought into a warm room, you will remember that the film of mist formed upon the glass is found by degrees to gather into drops, and trickles down the cold surface. Now the mist on the glass and the cloud in the sky are both formed of minute particles of water, separated by air. It is the running together of these particles which gives rise to these drops. In the one case, the drops run down the cold glass. In the other case, they fall as drops of rain through the air. Rain, therefore, is thus a further stage in the condensation of the aqueous vapor of the atmosphere. The minute particles of the cloud, as condensation proceeds, gather more moisture round them, until at last they form drops of water, too heavy to hang any longer suspended in the air. These then fall to the earth as rain-drops. But there is another important form in which the moisture of the clouds may descend to the surface of the earth. When the weather is cold enough, there fall to the ground not drops of rain, but flakes of snow. If you bring snow indoors, it soon melts into water. If you expose this water for a time it evaporates. Snow, water, and aqueous vapor are thus only different forms of the same substance. We say that water can exist in three forms—the gaseous, the liquid, and the solid. Snow is an example of the solid condition.