1354. The quantity of water which exists in the air, as vapour, varies with the temperature. In proportion as the weather is hotter, the quantity is greater. At 50ï¾¦ of Fahrenheit, air contains about 1/50 of its volume of vapour; and, as the specific gravity of vapour is to that of air nearly as 10 to 15, this is about 1/75 of its weight. At 100ï¾¦, supposing that there is a free communication with water, it contains about 1/14 part in volume, or 1/21 in weight. It is the condensation of vapour, by diminution of the temperature of the atmosphere, which is probably the principal cause of the formation of clouds, and of the deposition of dew, mist, snow, or hail.