=Aqueous Vapour= is always present in air, though the amount varies greatly. It is invisible in the ordinary condition, but by condensation becomes cloud or fog, rain, snow, or hail. The quantity of moisture present varies with the temperature of the air; the higher the temperature, the more water can be vaporised, without the point of saturation being reached. An increase of 27° Fahr. doubles the capacity of air for moisture. The amount of moisture that would saturate air at 50° Fahr. only gives 71 per cent. of the saturation amount at 60° Fahr. The amount of moisture is estimated by the _hygrometer_ (page 240). Air saturated with moisture at 32° Fahr., holds vapour equal to 1∕160 of its weight; at 59° it holds 1∕80, at 86° 1∕40, at 113° 1∕20, and at 140° 1∕10. =Ammonia= in normal air does not exceed one part in a million of air; but it is always present—either as free ammonia or as sulphate, chloride, carbonate, or sulphide of ammonia. From this source, plants derive some of the nitrogen they require as food; some also from the free nitrogen, which is fixed by certain microbes, growing in the nodules connected with the roots of peas, lentils, and other plants (page 274). Traces of nitrous and nitric acid are also present in the air, produced by the direct combination of nitrogen and oxygen occurring as the result of the electric spark during lightning. =Carbonic Acid= or carbon dioxide is always present in air, in the proportion of 3·36 to 4 parts in 10,000; but in impure air may be present in much larger amount. It is a heavy gas, incapable of supporting combustion, and therefore of supporting animal life. Being a heavy gas, it tends to accumulate where it is produced, as about lime-kilns by the heating of chalk. Thus CaCO₃ (chalk) (heated) = CaO (lime) + CO₂ (carbonic acid). Tramps have occasionally died of carbonic acid poisoning through sleeping near lime-kilns. It is produced by the oxidation of carbonaceous matters, hence in all ordinary combustion, in many cases of putrefaction and fermentation, and in the respiratory processes of all animals. Plants diminish the amount of carbonic acid in the atmosphere. Two processes occur in most plants: a process of respiration, as in animals; and a process of assimilation, by which the leaves and all other green parts of a plant under the influence of sunlight decompose the carbonic acid of the atmosphere, fixing its carbon and liberating its oxygen. Plants such as fungi, which are destitute of green colouring matter, cannot decompose carbonic acid; nor can any plants during the night. During the day green plants are air purifiers; during the night all plants vitiate the air to a slight extent. =The Air in Relation to Respiration.=—The oxygen of air is absolutely essential for the continuance of life. In every organised animal, _lungs_ or analogous organs are provided, in order to supply the necessary oxygen to the system, and to remove the impure air from it.