When common salt is dissolved in water, until the water can take up no more, the water is then said to be saturated with salt.
431. What proportion of water is air capable of sustaining in the form of vapour?
The amount of water held in suspension by the air averages the following proportion: one thousand cubic feet of air contain as much vapour as, were it condensed to water, would yield about two fifths of a pint.
But one thousand cubic feet of air are capable of holding half-a-pint of water; and this may be regarded as the point of saturation.
Thus, in a room ten feet square and ten feet high, the air, at the point of saturation, would hold in the form of vapour, half-a-pint of water. It must not be forgotten, however, that the point of saturation necessarily varies with the temperature of the air.
432. Why are cloudy days and nights not always wet?
Because the air has not reached the state of saturation.
433. Why does rain purify the air?
Because it produces motion in the particles of the air, by which they are intermixed. And it precipitates noxious vapours, and cleanses the face of the earth from unhealthy accumulations.
434. Why are mountainous localities more rainy than flat ones?