[AS.] When the temperature falls below 320 F., all superficial moisture changes into ice, and we have frost. Frost is one of the agents which play an important part in moulding the surface of the land, as it causes rocks to decay, and breaks up the materials of the soil. When rain falls it sinks into the pores of rocks, and soaks into the soil; and this moisture, in changing into ice, expands and pushes the particles of the rocks or of the soil more widely apart. Ten cubic feet of water give 11 cubic feet of ice. When a thaw sets in, the surface of the rock is loosened, and crumbles into soil, or is washed away by rivers to the sea, and the materials of the soil are broken up and rendered more fit to be made use of by plants. Black frost is where the cold turns vegetation black without hoar or white-frost.