DEW AND FROST

Tuapeka Times, Volume XXIV, Issue 1905, 18 June 1892, Page 2 (Supplement)

DEW AND FROST

Wonderful Productions of Nature that Puzzled the Ancient Philosophers*

Izitorcstng Facts Regarding Their History and tbe True Theory of Their Origin.

r: o Cnrionb Phenomena They Exhibit anrt ihe Erroneous Ideas Concerning Them.

To the ancient philosophers the appear-•v.m-c of dew and frost was a complete i. ystery, and numerous were the theories *. ivanced, by those scientifically inclined, to ; ■ count for their production, and the mi- ' resting phenomena they presented. Many, n is said, believed that tbe dew wns nothing i !se than tbe tears shed by the Almighty upon our earth, and they regarded it -as a ?;icred substance that possessed a number of wonderful and inexplicable properties. They »lqo attributed to it mnny strange events aad curious manifestations that occurred in those times, and some rubbed it over their bodies in order to protect themselves from the evil spirit, whose malignant influence was held responsible for all the marvellous and unaccountable operations of nature observed ia the earth and the heavens above.

A popular writer on scientific subjects, in lef erring to the formation of dew, says :

"There are many wonderful facts connected with the history of dew. It han attracted the attention of natural philosophers in all ages, but its true theory was never understood until quite recently. The. ancients imagined that dew came from the stars ; and the alchemists and physicians of the Middle Ages believed that dew distilled by night possessed penetrating and other wonderful properties. The ladies of those times sought to preserve their beauty by washing in dew, which they considered a ' celestial wash,' They collected it by placing upon, the grass heap 3of wool, upon the threads of which the magic drops clustered."

Among all the numerous truths •' stranger than fiction," that the study of natural science has revealed, there &T9 few more interesting and instructive than those relating to the phenomena of dew and frost. The late Pro. J. Dorman Steele, in his excellent work, " Fourteen Weeks in Physics," says :

" Dew was anciently thought to possess many wonderful properties. Baths in this precious liquid were said to conduce greatly to beauty. It was collected for this purpose, and for the use of the alchemists in their weird experiments, by spreading fleeces of wool upon the ground. Laurenß, a philosopher of the Middle Apes, claimed that dew was ethereal, bo that if we should fill a lark's egg with it and lay it out in the sun, immediately on the rising of that luminary, the egg wouid fly off into the air. This experiment is best performed with a goose's egg-"

There is considerable misapprehension among the general public concerning the nature and origin of dew and frost, it being commonly supposed by tbe great majority of people that dew actually falls from the eky, and how frequently do we hear the expression, "The dew ia falling fast." The fact is dew never falls, but is formed on the surface of the earth, aud always at the identical place where it is 6een by the observer. Dew ia simply the moisture of the air condensed by coraiug in contact with objects upon the ground that are colder than itself. The earth, heated during the day by the sun's rajs, at night cools oil and radiates a considerable portion of its heat into space. All bodies, left to themselves, will gradually become cool if previously heated. This is termed in natural philosophy, radiation, and it may be easily demonstrated by experiment. Take an iron ball, for instance, heat it to incandescence or until it becomes "red-hot." And then suspend it by a wire. In a little while it will part with all its heat, and feel quite cold to the touch. The heat has been thrown off, as it were, oc radiated, and not csrried away by the surrounding air, for precisely the same effect will be produced if this experiment is performed in a vacuum. Our earth is constantly radiating its heat, like the imaginary bail, in. all directions, and at night becomes so cool that the invisible vapor resting upon its surface is condensed into water and depcßited aa "dew." The temperature at which this takeß place is called the ♦• dew-point," as sndicated by an accurate and reliable thermometer. But thiß point iB by no means constant or invariable, since dew is deposited only when the air is saturated with vapor, and tbe amount of moisture required to saturate air of high temperature Is much greater thau for air of low temperature.

All bodies hare not the same capacity for radiating heat, but some cool much mora rapidity than others. Hence, with the same exposure, some bodies will be covered densely with dew, while others will remain perfectly dry. Dark objects have more dew deposited upon them than light ones, because they are better radiators, and, therefore, part with a larger amount of heat. This fact may also be demonstrated by placing a black woollen cloth and a white linen or cotton one upon the ground over night. Grass, the leaves of trees, wood, etc., radiate heat very freely ; but polished metals, smooth stones and cotton cloth part with their heat more slowly, bo that the former of these substances will be completely drenched with dew, while the latter, in the same situation, will be comparatively dry.

The amount of due formed on any night is greatly influenced by the condition of the jttmosphere. More dew ih formed on clear evenings than when the sky is obscured by clouds, because they act as a shield, and reSect back the heat radiated by the earth into space, besides furnishing that received by them during the day, which prevents the earth from cooling off enough to produce a large deposit of dew. Bat when the sky is free from cloudß the earth radiates a larger amount of heat, as there is nothing to intercept that thrown off from ita surface. This .s the reason why we always have colder nights when no clouds obscure the sky. The formation of dew is prevented by a strong Dreess, which removes the layers of moisture resting upon the ground before they haT6 time to cool down to the •• dew point;" A. slight breeze, however, is favorable to the production of dew, as it replaces ths layer of air from which water has been daposited by another containing more moisture. No dew may ever be expected when the sky it obscured by clouds.or when there iB a stroag wind.

The moisture that appears on the outside of a pitcher when it contains very cold water is nothing but dew, and is produced by the same cause explained above. Tfaii dampness, frequently observed upon a cold pitcher, even during the hottest weather in summer, is commonly referred to by Baying "the pitcher is sweating;" and in this aimple illustration we are permitted to xecogaise before our eyes, on a small scale, the same interesting process of dow formation that takes place over the surface of the great eazth. Dew is alluded to in the writings of many ancivat philosophers, and the <Sible contains a quaint passage in regard to Qideou'B fleece. (See Judges vi.. 37-40.)

Many portions of the earth's surface are greatly benefitted by the production of dew, and in some parts of the world nearly all tbe moisture that the earth ever receives comes in this form. This is particularly true of some portions of Egypt and Arabia. The deposition of dew sometimes commences before sunset. It continues at all sours of the night, provided the weather resains favorable ; but more dew is formed after inidnigtt than before, and the process of dew formation sometimes costiaaes even after i^oriße. In places sheltered from the «un, but open to a considerable portion of the sky, dew is Bometimes deposited upon Krass even at mid-day. Only a small amount of dew is deposited in cities, because most of the- objects there found are poorer radiators than the leaves of vegetables, and also because the heat of the city •s always greater thau that of the surroundjig country. Prof. S teele bs js :

"The grass at night becoming cooled by radiation, condenses upon its Burface the vapor of the air. Daw wiil gather most, freely upon the best radiators, as they will the soonest become cool. Thus grass, leaves, etc., which need the most, get the most. It will not form on windy nights, because the air is constantly changing and does not become cool enough to deposit its moisture. In tropical regions the nocturnal radiation is often so great as to foim ice. In Bengal water is exposed for this purpose in Bhaliow earthen dishes resting on ice straw. The most dew collects on a clear, cloudless night. In Chili, Arabia, etc., by its abundance, itsupplics the place of rain."

As a certain amount of cooling deposits moisture from the air in the form of dew, more still freezes it, thus producing what is known as white or hoar-froßt ; and hence "frost" is simply frozen dew. When dew has been deposited, if the temperature of the air falls below the freezing point of water — which ia 32dcgs. above zero by the common house theiraometer — then dew, consisting only of condensed watery vapors, will freeze and produce frost. All persons have seen this beautiful white mantle, and nearly every child has doubtless heard of " Jack Frost." This is the very substance that frequently appears in such fantastic forms over our window-panes on cold winter mornings. Since plants sometimes become cooled by radiation from 12 to 15 degs. below the temperature of the surrounding air, a frost may occur even though a thermometer only a few feet above the ground does not fall- to 32dega. During a clear and still night, whan a thermometer six feet above the ground sinks to 36dega., a very heavy frost may be expected, and a slight frost may occur when the same thermometer sinks only to 47degs. A peculiar kind of hoar-frost is sometimes observed whep. a warm wind follows a period, of severe cold weather. Stone buildings are then often covered with an incrustation of minute crystals caused by the low temperature of the Btone, which condenses and then freezes the moisture of the air.

Prof. Loomis thus explains how plants may be protected from frost :

" Whatever prevents tho radiation of heat serves also to check the formation of hoar-frost. Baring the cold nights of spring pla<nts which are sheltered Dy treei are less liable to be injured by frost than those which are fully exposed, and a thin covering of cloth or straw will generally afford entire protection. A garden may frequently be saved from injury by kindling a small fire, which shall develop the plants in a cloud of smoke. Fogs and clouds also protect vegetation from tho effect! oi frost." .