Mists. There is a certain quantity of water existing in a purely gaseous and invisible form in all states of the atmosphere. The higher the temperature of the air the more water in this form it. is capable of holding in solution. If the air is fully saturated with moisture (which it always is over the course of a river), and the temperature be reduced, the water in it is condensed in the form of mist, which has been aptly, though quaintly, called " water-dust." The radiation of heat from off' the surface of water is almost always greater than off the surface of land. Thus, alter sunset, the water rapidly cooling, cools the air immediately in contact with it, in which the water hitherto gaseous becomes condensed, and visible as mist. This, in turn, cools the air above it.