78 



SCIENTIFIC NEAVS. 



[Jan. 27, 



act ; in front it is covered with painted canvass, which 

 gives it the appearance of an ordinary drop-scene. We 

 trust that similar screens will be introduced at all the 

 theatres in England. 



HOAR-FROST AND FOG. 



MR. John Aitken, ot Darroch, Falkirk, has contributed 

 totheProceedingsof theRoyalSociety of Edinburgh, 

 an interesting note on the formation of hoar-frost. Hoar- 

 frost is generally described as frozen dew, and is sup- 

 posed to be deposited in the same manner and under the 

 same conditions as dew,* the only difference being in 

 the temperature at which it is deposited. Though in a 

 general way this may be so, yet there are certain differ- 

 ences in the conditions and the manner in which the 

 vapour is condensed at the various temperatures, which 

 he has brought before the Society. 



If a surface, such as a sheet cf glass, be exposed horizon- 

 tally near the ground on a dewy night, the windward edges 

 are generally found dry. This indicates that the air itself is 

 not cooled to the dew point, though the surfaces of bodies 

 exposed to radiation are, and the air has to travel some 

 distance over the cold surface before its temperature is 

 reduced to the dew-point. If, however, the same surface 

 be examined when the temperature is low enough to 

 cause the deposited moisture to form hoar-frost, a 

 marked difference is frequently found. The sheet of 

 glass is generally not only covered with the deposited 

 vapour up to the windward edges, but the deposit is 

 heaviest along these edges, the ice-crystals growing 

 furthest out in that direction. This peculiarity in the 

 deposition of hoar-frost may also be observed on almost 

 all objects — such as branches of trees, iron fences, &c. 

 The heaviest deposit will often be found on the wind- 

 ward side, and not on the top, where it might be ex- 

 pected, owing to the stronger radiation from that surface. 



He has investigated the cause of the difference be- 

 tween the deposits of dew and hoar-frost. The dryness 

 of that part of the dewed surface where the air first 

 touches it is caused by the air not being saturated, and 

 requiring to travel some distance over the cold surface 

 before it is cooled below its dew-point. When, however, 

 frost is forming, the air seems generally to act as if it 

 were supersaturated ; the crystals growing most towards 

 the wind seems to indicate that the air does not require 

 to be cooled before it deposits its moisture. Now, under 

 ordinary conditions, it is impossible for the air to be 

 supersaturated. Owing to the vast amount of dust in 

 the air, there is always plenty of free surface present to 

 prevent supersaturation, so long as the temperature is 

 above 32" Fahr. But when the temperature falls below 

 the freezing point, a much more complicated state of 

 matters is presented. 



It has been found that ice and water have the same 

 temperature at 32°, under a pressure of 4-6 mm. ; but 

 when the pressure is about 3-20 mm. the water is at a 

 temperature of about 23" while the ice is about 24°. Thus 

 with the reduced pressure the water is about a degree 

 colder than the ice. If then the ice be cooled to the same 

 temperature as the water, itg vapour-pressure will be 

 less than that of the water, and if a water-surface and an 

 ice-surface be near each other at the same temperature, 

 vapour will tend to pass from the water to the ice. As 

 the vapour-pressure of the water is higher than that of 



* Scientific News, Vol. I., Old Series, p, 67. 



ice, the air which is saturated to a water-surface is super- 

 saturated to an ice-surface. 



Something like this seems to take place when hoar- 

 frost is forming. When the air is cooled, condensation 

 takes place on the dust particles, resulting in a fog. This 

 moisture condensed in the air seems always to keep the 

 liquid form ; though during frosty weather the fog is in- 

 tensely cold. In the fogs formed low down in the atmo- 

 sphere there are no optical or other phenomena such as 

 might be expected if the particles were frozen. Thin 

 films and small drops seem difficult to freeze. It is not 

 unusual to see a liquid film on the night-radiation ther- 

 mometer, although the instrument is reading many 

 degrees below the freezing-point. Accordingly fog par- 

 ticles in frosty weather may be found liquid, and the 

 pressure of the vapour in the air, as it corresponds to that 

 of a liquid surface, will be greater than that of an ice one 

 at the same temperature. Under these conditions, the 

 air will rapidly unburden itself of part of its vapour when 

 it comes into contact with an ice-surface. This is why 

 hoar-frost grows in the direction from which the air is 

 moving; because the air, being supersaturated, unburdens 

 itself on the first ice-surface with which it comes into con- 

 tact. On the other hand, when the dew is forming,it requires 

 to be brought into a condition to cause it to give up its 

 vapour, which is done by travelling over the cold surface. 

 Mr. Aitken has never observed a heavy deposit of 

 hoar-frost when the sky is clear, or in those conditions in 

 which the heaviest deposits of dew are formed. On all 

 those occasions on which trees and every exposed sur- 

 face become clothed in hoar-frost, the transformation is 

 always accomplished in a thick and foggy atmosphere. 

 This bears out the explanation which he has given. On 

 thick and foggy nights, little or no dew could be de- 

 posited, because the radiation would be checked by the 

 fog. But on these very nights heavy deposits of hoar- 

 frost are most likely to be formed, because the air has a 

 large amount of vapour in it. Further, while dew re- 

 quires that the surface on which it is deposited be cooled 

 by radiation, this is not so necessary in the formation ot 

 hoar-frost, because the fog-particles radiate and cool the 

 air to the saturated temperature of vapour at a water- 

 surface, and the passing air discharges part of its vapour 

 on all ice crystals or other nuclei with which it comes in 

 contact ; the passing air at the same time absorbs the 

 heat of crystallisation, while the heat of condensation is 

 balanced by the heat absorbed by the evaporation from 

 the water particles. Accordingly hoar-frosty nights do 

 not correspond to dewy nights (when the temperature is 

 higher), but to those nights when every object is wet and 

 dripping with deposited fog-particles. 



Special Metal for Bearings. — Owing to the great rise 

 in the price of tin, Babbitt's metal, when made according to 

 the original formula, now costs proportionately more. Tay- 

 lor's metal, made by Messrs. Osborne and.Co., is not, how- 

 ever, affected in price, and it answers the same purpose as 

 the Babbitt's. 



Celluloid for Coating Ships. — A new application has 

 been found for celluloid, tor, according to the Annales Indus- 

 trielles, it can be used as a sheathing for ships instead of 

 copper. Sheets of celluloid after being attached to vessels 

 for five to six months were, on examination, found to be un- 

 altered and quite free from the marine vegetation growing 

 freely on the unprotected parts of the ships. The celluloid 

 can be supplied in very thin sheets, its colour is prac- 

 tically unchangeable, and its impermeability is also in its 

 favour. 



