FROST, the moisture in the atmosphere crystallized or congealed by the cold, upon the earth's surface, or upon various objects and surfaces situated or existing upon it, as grasses, shrubs, trees, window panes, etc. The various phenomena of hoar frost, window frost, etc., grouped tinder this head, occur over a large portion of the land surface of the earth. In the United States, hoar frosts often occur during the spring and autumn months, over the whole northern portion of the country, and more rarely also in the Southern States, sometimes causing much damage by freezing and injuring the young corn sprouts and early fruits and vegetables. Frosts occur only during calm, cold nights when the temperature falls below 32° F. In the United States the meteorological condi tions usually preceding the formation of frosts are northerly winds, accompanied by high ba rometer, and especially the coincidence of these conditions with the near approach, or passing, of a storm from the west or southwest. The for mation of hoar frost depends in some degree upon surface topography and local causes, oc curring much more frequently within the deep valleys leading down from mountain heights, than in broader valleys or level regions. In the former, during calm cold nights, the cold air of the hill and mountain tops, by virtue of its greater specific gravity, flows down and mixes with, or flows underneath and replaces, the warmer, lighter air of the valleys, thus furnish ing the conditions favorable to frost formation. The cooling of objects by radiation of heat, and by the evaporation of moisture from them, greatly facilitates the formation of frost. Frost crystallizations exhibit a wonderful variety, both of form and structure. The formation of each of the various types seems to depend upon a great number of meteorological and other condi tions, some of them obscure. The temperature of the air, its electric condition, humidity, etc., and also the nature of the substances upon which they form, each seem to exert an influence in determining and modifying their form and structure. Two principal types of hoar frost occur,—the columnar and the tabular. Com monly, both varieties do not occur simul taneously, but on a given night one or the other type will greatly predominate and form the bulk of the crystals. Frost crystallizations in general greatly resemble those of snow, but be cause their development is usually restricted in one or more directions by the objects or surfaces upon which they form, the resemblance is seg mentary, rather than complete. In general, columnar forms vary from similar snow crystal lizations, by virtue of their hollow cylindrical, or cup-like character, and by often attaining to much greater dimensions. Sometimes, during extreme cold, such forms attain a length of many Inches. Tabular forms rarely attain perfect sym metry, but exhibit within them air tubes and inclusions, assume crystal forms possessing both close and open structure, and develop upon the same extremely thin plane as do similar snow crystals. As commonly deposited in spring or autumn, they do not usually greatly exceed in size similar snow forms, but during intense and prolonged cold, as in winter, they attain much greater dimensions. A very beautiful effect is sometimes produced by the deposition upon the trees, shrubs, etc., of a heavy coat of hoar frost. Each limb and leaf and delicate twig is trans formed and beautified, and presents a white ap pearance, as though frosted with silver. During zero weather, large and delicately formed branch ing tabular crystals, and long, icy needles, form in beautiful pendent clusters upon, and depend from, the rafters and timbers of barns, etc.,
close to where domestic animals are kept ; and also upon ferns and similar plants overhanging icy terraces or ice-covered pools. Similar forms also form directly upon or project from icy surfaces. Even the clouds furnish their quota of frost crystallizations. When low-lying clouds enshroud mountain tops covered by forests, they often deposit a portion of their moisture upon the branches of the trees, commonly in the form of long, granular or fibrous needle-shaped crys tals. Fogs, when they occur during hoar frost formation, usually deposit moisture upon the forming crystals in granular form. The most beautiful and varied frost crystallizations are those that form upon the window panes of dwelling houses, etc., in arctic or temperate zones. These fairy-like creations, seemingly in imitation of leaves, feathers, ferns, trees, starry firmaments, tropical forest effects, etc., occur as three distinct entities : the granular, the crys talline, and the membraneous.. The latter va riety forms only in heated rooms, upon window panes covered with an uncongealed film of water, as a dew-like condensation of moisture. It oc curs most frequently in the form of long, curv ing, feather-like forms, or as an exceedingly delicate membranaceous-like network of diverg ing and coalescing lines. It is due to a process of crystallization that takes place during the conversion of a film of water into ice. The crys talline variety of window frost forms only upon windowpanes that are free from water in liquid form. Crystals of this class assume branching, star-like forms (often as four- or six-pointed branching stars), curving filaments, fibrous crys tallizations, and those resembling sea-moss, long serrated lines, etc. Many of these are very beau. tiful and interesting. Some of them develop within minute striations in the surface of the glass and will reappear in the same identical posi tions upon a given pane, with each renewal. When identical meteorological and other condi tions recur again and again, the types of frost coexistent with each will, in general, recur simultaneously with them. During zero weather, if conditions are favorable, the formation and growth of these beautiful frost creations takes place very rapidly. A beautiful and absorbingly interesting experiment consists in melting a heavy coat of window frost off a portion of a window pane (by placing an oil lamp close to it). Only the central portion of the pane should be cleared of all moisture; around this a film of water should be left upon the glass. Soon after the lamp is removed the feather-like mem braneous frost will form around the outer edges of the film of water, and quickly radiate in beau tiful curves toward the centre of the pane. They stop instantly when they reach the clear glass. Soon minute and delicate serrated crys tal lines, or tiny crystal stars, appear upon the clear glass space, and slowly develop, and usually coexistent with them a thin film of granular texture will be laid down upon portions of the clear glass. The latter is not usually deposited in slow progressive order, but in intermittent order. Large spaces of the clear glass are often covered simultaneously, by a succession of auro ral-like flashes; each flash, in the twinkling of an eye, spreading a thingranular film upon un occupied portions of the glass. Singularly enough, the granular deposit does not form near where the true crystalline frost is; the latter repels the former and prevents its forma tion upon the spaces immediately surrounding it.