After sunset, when the sky is clear, you know that the grass gets wet with dew. In the morning you may see mists hanging over woods, and streams, and hills, and gradually melting away as the sun mounts in the sky. At all times of the year you may watch how clouds form and dissolve, and form again, ever changing their size and shape as they move through the air. Now these are all examples of the condensation of vapor. Let us see how the process takes place. Condensation, as we have seen, results from a cooling of the air. When vapor is condensed, it does not at once take the form of running water. The cold glass brought into the warm room has first a fine film of mist formed upon it, and then by degrees the clear drops of water come. In reality mist is made up of exceedingly minute particles of water, and it is the running together of these which makes the larger drops. So in nature on the great scale, when condensation occurs the vapor first appears as a fine mist. This is always the result of cooling; so that, whenever you see a mist or cloud forming, you may conclude that the air in which it lies is being cooled. Dew is the name given to the wetness which we notice appearing in the evening, or at night, upon grass, leaves, or stones, or even sometimes on our hair. In the morning you have, no doubt, often watched the little dewdrops sparkling upon the foliage and the delicate threads of gossamer. Now this wetness does not come out of the leaves or stones, nor out of your hair. It is all derived from the air by condensation, exactly as we see the film of mist form upon the cold tumbler in the warm moist air of a room. In fact, that film of mist was really dew, and all dew is formed in the same way, and from the same cause. At night, when the sky is clear, the earth radiates heat rapidly; that is to say, it gives off into cold space a great part of the heat which it has received from the sun during the day. Its surface consequently becomes cold, as you may have felt when you put your hand upon leaves or stones after nightfall. The layer of air next the cooled ground is chilled below its point of condensation, and the excess of vapor is deposited as dew upon the grass, twigs, stones, and other objects. Hence it is that the temperature at which this condensation begins to take place is called the dew-point.