258 



KNOWLEDGE. 



[November, 1901. 



at a spot a few miles from Inverness. The shock is said 

 to have been felt at Dunbar, Wick, Tobermory, in the 

 island of Mull, and at Aberdeen and Peterhead. Its 

 disturbed area must therefore extend over the greater 

 part of Scotland ; and, since it includes Edinburgh 

 (though no record has yet come from tliat city), it is 

 strange that the shock should not have been registered 

 by the seismograph erected in the Royal Observatoi-y. 

 The explanation probably is that, the boom of the 

 pendulum being placed in a north and sotith plane, it 

 would only be affected by movements with an east and 

 west component. — C. D. 



-♦■ 



THE ALCHEMY OF HOAR-FROST. 



By Arthur H. Bell. 



Cooled surfaces are Natiu-e's plates, upon which she 

 etches some of her most beautiful pictures. In this 

 artistic work she employs many materials, but her 

 choicest effects are obtained through the medium of 

 hoar-frost. Commonly, hoar-frost is described as being 

 merely frozen moisture, but this is not an adequate 

 description of an agent that has the power of adorning 

 in a few hours such prosaic objects as gate-posts and 

 dust-bins with all the trappings of fairy land. Moisture 

 is indeed the fabric out of which all this featheiy white- 

 ness is built up, but, although it seems sometimes as if 

 it is distributed in a vei-y capricious manner, there are 

 nevertheless certain definite circumstances which cause 

 the hoar-frost to settle down on some surfaces rather 

 than on others. 



On any cold and frosty morning it will usually be 

 found that those surfaces that are the best radiatoi-s 

 of heat are also those that are most successful in collect^ 

 mg hoar-frost. It is not always realized, however, that, 

 all objects are continually radiating heat:, so that no 

 matter how much they may receive from the sun, they 

 are constantly tr-ying to get rid of it.. A fern leaf, or 

 a stone, may perhaps receive generous supplies of heat 

 during the day, but as soon as night comes it huiTies 

 to spend or radiate it, and the object that is C)uickest 

 at tbis work will the soonest become covered in hoar- 

 frost. Everyone has observed how the moisture from 

 the air will settle on the outside of a glass of cold water 

 brought suddenly into a warm room. A similar process 

 takes place in the open air, so that as the currents of 

 moist air travel across surfaces that are very cold, they 

 pay tribute in drops of vapour, which in wann weather 

 lake the form of dew, and in cold of hoar-frost. 

 Moisture, therefore, plays a very important part in the 

 development of these hoar-frost. ))ictures ; but there must 

 not be too. much of it. Some of the most delicate designs 

 occur during the prevalence of mist and haze, and in 

 towns especially it is no uncommon thing for a choking 

 brumous fog to be in some degree comjiensated for by 

 a subsequent display of copious hoar-frost. As regards 

 discovering what kind of surfaces are best adapted for 

 collecting hoar-frost it may, in passing, be said that a 

 very instructive and entertaining series of observations 

 may be obtained by exposing to the frost cups, dishes, 

 tumblers, saucers, and other glass and china ware, which 

 will be • found to accumula.te hoar-frost in a vai-ying 

 degree. A brief contemplation of these differences will 

 clearly demonstrate the fact that it is those objects 

 that cool the quickest that make the speediest responses 

 to the alchemy of frost. 



In certain parts of the world agriculturists protect 

 their crops from damage by frost by setting light to 

 heaps of rubbish, thus producing cloucls of smoke, which 



check the radiation of heat from the surface of the 

 ground. By this means frosts aj-e warded off and the 

 life of susceptible plants is prolonged. A similar thing 

 happens when real clouds float overhead, it being a 

 common experience that no dew or hoar-frost forms 

 when the night is cloudy. In other words, a canopy 

 of clouds acts towards the earth as an overcoat and 

 prevents the loss of the heat which it received from the 

 sun during the day ; for no sooner does this heat attempt 

 to escape into space than the clouds reflect it earthwards 

 again, and they form indeed a veritable trap for sun- 

 beams. But when the air is damp and the stars ai-e 

 shining brightly, the thermometer at the same time 

 being ten or more degi'ees below the freezing point, 

 everything will be dusted over with fragile flowers of 

 frost, and more especially if there happens to be little 

 or no wind. 



During very many yeai's it has been a popular super- 

 stition that it is injurious to sleep with the moon shining 

 on one's head. From what has already been said as to 

 the way in wliich moisture promistly settles on all 

 objects that are radiating their heat quickly, it will be 

 gathered that it is not so much tlie moonbeEuns that 

 work the mischief as the loss of warmth and the 

 deposition of moisture which falls on all surfaces exposed 

 to the sky on cold and cloudless nights, when dew and 

 hoar-frost ai'o most abundant. 



It is, further, not commonly realized that the 

 atmosphere acts as regards moisture very like a. sponge. 

 According to this conception of the case the air is not 

 only able to absorb large qua.ntities of water, but it is 

 also able to retain it; and it is only when something 

 happens to squeeze the atmosphere, as the process may 

 be termed, that the hold on this moisture is relinquished. 

 This squeezing of the atmosphere takes place whenever 

 there is a fall in temperature, this being the great agent 

 or force that precipitates the moisture from the air and 

 causes it to take the form of rain, hail, snow, fog, dew, 

 or hoar-frost; these various fonns being regulated by 

 the condition of the atmosphere. Although the air parts 

 so readily with it,s stores of moisture when the tem- 

 perature falls, it is to b© observed that an increase of 

 tcmperatui'c greatly enlarges its capacity for moisture. 

 A cubic foot of air having a temperature of 32 deg. can 

 accommodate only 213 grains of moisture; but suppos- 

 ing the temperature to be increased to 72 deg; there 

 would then be room for 8"47 grains. It will therefore 

 readily be understood that in looking out for copious 

 displays of hoar-frost, the best pictures will be observed 

 if during moist weather a body of air having a high 

 temperature is suddenly reduced to the freezing point. 

 "One of the best ways of keeping a jar of water cool 

 is to wrap a damp cloth round it; the evaporation of 

 the moisture producing loss of heat. In hot climates 

 this circumstance is, indeed, made of practical service 

 as regards the manufacture of ice, for, so- intense is 

 nocturnal evaporation of moistuie, that it is found that if 

 water is placed in shallow poi'ous pans overnight, there 

 is a welcome supply of ice in the morning. When, 

 therefore, moisture is evaporating into the atmosphere 

 i.here' is always a loss of heat, so that the greater the 

 amount of vapour passing into the air the greater the 

 amount of heat used up. 



It is an interesting fact that when hoar-frost, or 

 dew, or any of the other children of acjueous vapour, 

 spring into being, this heat reappears, or, as it is some- 

 times conveniently described, latent heat is set free. 

 As regai-ds rainfall the lunouut of heat liberated is, 

 of course, greater than is the case with hoar-frost. A 



