Aqueous vapor is contained in the air in quantities varying in different localities and at different times, and depending mainly on the temperature of the air. Air at a given temperature cannot contain more than a certain quantity of moisture in solution; and when it has taken up its maximum quantity, it is said to be saturated with aqueous moisture. The higher the temperature of the air the more water can it retain as vapor; and when air saturated with moisture is cooled, the water is deposited in liquid form in very small globules, forming a mist, fog, or cloud. This is the cause of the fall of rain, snow, and hail; when warm air heavily laden with moisture from the ocean passes into a higher and colder position, or meets with a stratum of air of lower temperature, it cannot any longer retain so much aqueous vapor, and a large quantity assumes a liquid form, falling as rain when the temperature is above the freezing-point, or crystallizing as snowflakes if the temperature is below that point. Hail is caused by the congelation of raindrops in passing through a stratum of air below the freezing-point. The deposition of dew is caused by the rapid cooling of the earth's surface by radiation after sunset, and by the consequent cooling of the air near the ground below the temperature at which it begins to deposit moisture. In general the air contains from 50 to 70 per cent. of aqueous vapor of the quantities necessary to saturate it. If the quantity be not within these limits the air is either unpleasantly dry or unpleasantly moist. The air contains, besides the gases of oxygen and nitrogen, carbonic acid, ammonia, accidental impurities, and volatile organic matter, which latter is the most important, as it probably influences to a great extent the healthfulness of the special situation. We become aware of the existence of such organic putrescent substances when entering a crowded room from the fresh air; and it is probable that the well-known unhealthiness of marshy and other districts is owing to the presence of some organic impurities. We may have occasion to refer to this when speaking of the deluge, etc. CHAPTER III. THE CHEMICAL ASPECT. By the word chemistry we understand the science which investigates the composition of all material substances, taking them apart or separating them, by a chemical process, and discovers the nature and properties of the minutest particle. These small particles have received the name, elements or elementary substances; that term is applied in chemistry to those forms of matter which have hitherto resisted all attempt to decompose them.