Licence: Public Domain Mark
Credit: The atmosphere in relation to human life and health. Source: Wellcome Collection.
Provider: This material has been provided by the Augustus C. Long Health Sciences Library at Columbia University and Columbia University Libraries/Information Services, through the Medical Heritage Library. The original may be consulted at the the Augustus C. Long Health Sciences Library at Columbia University and Columbia University.
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![■writer lias been led by many personal observations to the conclusion tLat the crystals are differently developed according to (1) the amount of dust or nuclei in the air, (2) the electric state, (3) the humidity of the stratum where they have their origin and of the lower strata, and (3) the suddenness or slowness of their growth. He found that in a clear air on a hill crystals on vegetation were clearer, simpler, and more glassy than in the rather foggy neighboring valley; that in the neigh- borhood (10 miles) of London, where the air was smoky, the crystals on trees were very much more feathery, branching, and ojjaque, and yielded smoky water on melting. The upper air varies greatly in the amount of contained dust nuclei, in free electricity, and in differences of temperature between strata. A moist southerly wind beating back a cold northeast wind in England generally yields broad, heavy, irregu- lar, conglomerated flakes; a dry gentle wind, with uniform conditions, yields regular crystals, small and thin; a very dry and cold air in the early days of a severe frost sometimes gives showers of pellets of vari- ous sizes, roughly hexagonal or polygonal, very dense, thick, opaque, and like a number of superposed plates. In March, and sometimes in April, a soft hail or dense pellets of snow fall in showers with a north- east or north wind, and dry air, the showers alternating with bright sunshine. At great heights in the Alps, the snow in winter is small and powdery; in summer the flakes are much larger. Hail is often the result of a sudden condensation of very warm, moist air by great reduction of temperature at a great height. The dust nuclei are soon all occupied by moisture condensed upon them, and as the vapor falls to and below saturation point in a high column, it has not sufficient nuclei on which to condense in cloudy form, and precipi- tation takes place at a great rate, either on the cloud globules or on the snow crystals which fall through from the upper part of the cloud. Since the whole or a great portion of the column of the topmost cloud is below the freezing point, the globules as they come in contact with the falling crystals instantly freeze, and so the crystal grows and falls ever faster, accumulating bands of ice and snowy ]3articles according as the air is clear and saturated, or else densely cloudy, through which it passes. The electric charge being much denser comparatively on a large drop or crystal than on a small one, and the vapor pressure being less, the hailstones grow very quickly, and since they fall raioidly through very thick clouds, they add much ice by mere impact at their base. The radial structure so often observed indicates the origiu of the hailstone from a radial snowflake or hexagonal plate. Hailstones of large size are produced in circumstances of great electric disturbance. Sometimes a hailstone has been found with finely developed hexag- onal ice crystals growing like stalactites from a matrix. Possibly the attachment of a flat hexagonal crystal at a certain stage in the fall of the hailstone and the action of electricity in tbe rapid passage through the air are sufficient to account for these large ice crystals, but they](https://iiif.wellcomecollection.org/image/b21208724_0136.jp2/full/800%2C/0/default.jpg)
