The fact is, then, that whenever a cloud overhangs, _rain is falling_, though it may not reach the earth on account of the dryness of the stratum of air below the cloud, and the heat of the air over the earth. So that on a summer day, with the gold-fringed, fleecy clouds sailing overhead, it is really raining; but the drops, being very small, evaporate long before reaching the earth. As Ariel sings at the end of "The Tempest" of Shakespeare, "The rain, it raineth every day." It rains, but much of the melting of the clouds is reproduced by a wonderful circularity--the moisture evaporating, seizing other dust-particles, forming cloud-particles, falling again, and so on _ad infinitum_, during the existing circumstances. CHAPTER XII HAZE What is haze? The dictionary says, "a fog." Well, haze is _not_ a fog. In a fog, the dust-particles in the air have been fully clothed with water-vapour; in a haze, the process of condensation has been arrested. Cloudy condensation is changed to haze by the reduction of its humidity. Dr. Aitken invented a simple apparatus for testing the condensing power of dust, and observing if water-vapour condensed on the deposited dust in unsaturated air. The dust from the air has first to be collected. This is done by placing a glass plate vertically, and in close contact with one of the panes of glass in the window, by means of a little india-rubber solution. The plate being thus rendered colder than the air in the room, the dust is deposited on it. Construct a rectangular box, with a square bottom, 1-1/2 inches a side and 3/4 inch deep, and open at the top. Cover the top edge of the box with a thickness of india-rubber. Place the dusty plate--a square glass mirror, 4 inches a side--on the top of the india-rubber, and hold it down by spring catches, so as to make the box water-tight. The box has been provided with two pipes, one for taking in water and the other for taking away the overflow, with the bulb of a thermometer in the centre. Clean the dust carefully off one half of the mirror, so that one half of the glass covering the box is clean and the other half dusty. Pour cold water through the pipe into the box, so as to lower the temperature of the mirror, and carefully observe when condensation begins on the clean part and on the dusty part, taking a note of the difference of temperature. The condensation of the water-vapour will appear on the dust-particles before coming down to the natural dew-point temperature of the clean glass. And the difference between the two temperatures indicates the temperature above the dew-point at which the dust has condensed the water-vapour.