The explanation of the formation of dew and hoar-frost which Dr Wells published about seventy years ago, has been almost universally accepted as satisfactory ever since. Shortly stated, Dr Wells’ ‘Theory of Dew’ is as follows: Air always contains a certain amount of moisture in the form of invisible vapour. The hotter the air is, the more vapour will it contain. Thus, during a warm day, a good deal of moisture passes into the air; and when the temperature falls in the evening, some of it is deposited as a fine mist. But even when this mist does not appear, dew is formed. As soon as the sun is down, especially if it is a clear evening, the grass, trees, shrubs, and even the soil itself rapidly get cooled by radiating into space the heat which they contain. These cooled bodies in turn cool the warm air above them, and this causes it to deposit more or less of its moisture, which appears either as a film or in minute drops of dew. The points of the grass, small twigs, and all other good radiating surfaces are cooled the most; and accordingly we find the dewdrops most abundant on these bodies; whilst on metal or hard stone surfaces, which are poor radiators, we seldom or never find any dew. A clear, cloudless sky, which promotes radiation, is always favourable to the formation of dew; but on cloudy nights, little is formed, because the clouds return the heat radiated from the earth. Hoar-frost is only dew deposited on bodies cooled below the freezing-point. It is formed in winter when the temperature of the air during the day is only a little over this point. At night, the grass and ground are soon cooled below thirty-two degrees Fahrenheit, and what moisture is deposited appears as minute ice-crystals or hoar-frost. Many experiments can be cited which tend to strengthen and confirm this explanation. Thus, every one is familiar with the fact of glass bottles, mirrors, &c., being covered with moisture on being brought into a warm room. The same thing happens with a cold cabbage leaf, or with a bundle of vegetables or a bunch of flowers. On a cold night, the windows of a warm room soon get dimmed. Still more striking is a phenomenon which frequently occurs in countries where the temperature is much below the freezing-point in winter. The houses are well heated, and if a number of people are together, as in a ballroom, the air soon becomes moisture-laden. If the ventilation is not over-good, it may happen that a door or window will be opened. With the rush of cold air from without, the merry-makers are often alarmed by being suddenly covered with hoar-frost, or sometimes even a shower of snow. This does not come from the outside, as it occurs most readily on cold, clear, starlit nights. It is the moisture of the air of the room suddenly cooled below freezing-point that appears as snow or hoar-frost. Many similar experiments may be noticed, all of which are satisfactorily accounted for on Dr Wells’ theory.