FOG AND DEW PONDS.
(To THE EDITOR OF THE "SPECTATOR."] Srn,—I have read with interest the article on "Fog and Dew Ponds" which appeared in the Spectator of November 16th, and would venture to suggest the following as a possible explanation of the formation of dew ponds. It is believed by some scientists that fog and mist are formed by the conden- cation of water vapour on minute particles of dust which float in the air. In this manner small spheres of water are formed, and become visible because they scatter light. The anion of several of these spheres produces drops of water, which fall as rain or collect on the nearest object. Experience proves that a sheet of water collects these minute lpheres much more readily than any other surface. The whole of the mist as soon as it is formed may be in a state of strain, each sphere attracting and being attracted by those in its imme- diate neighbourhood. A condition of equilibrium results from these mutual attractions, for supposing all the particles to be equal in size and separated by equal intervals, any one particle would be equally attracted by those on all sides of it and would, therefore, remain stationary. Such would be the state of things were the particles all equal in size and at equal distances apart; but such a theoretically perfect state of conditions does not obtain in practice. The balance would be slightly upset by a difference in size of the particles and variation of the separating intervals, and the comparatively slow condensation of mist into rain would take place. The balance would be rudely upset by a sheet of water, hence the particles in the immediate neighbourhood of a pond would be- attracted by and condense Oil to it. A tree close to the pond would first become damp_ by the slow condensation of the mist on to it. This condensed moisture would attract more till drops are formed, and these, in falling from• leaf to leaf and from twig to twig or to the ground, would attract still more. The force setting up the strain in the mist may be the attraction due to gravitation, aided or counter. acted by electrical attraction or repulsion. There is nearly always some electricity in the atmosphere, and particles of mist charged with electricity would be attracted by anything in contact with the earth, and would discharge their electricity and coalesce with other drops at the same time. Water being a good conductor of electricity would discharge their electricity quickly, and would therefore attract a greater number than a dry surface. Pointed objects also cause a discharge of electricity from the atmosphere to the air; hence leaves, twigs, and grass attract more mist, and therefore condense more dew, than flat surfaces. The state of strain in a mobile medium mentioned above may be imitated artificially. If some water is boiled in a glass flask holding about a quart, and either nitrate of potash or sulphate of soda added in small quantities until no more will dissolve, the solution is said to be supersaturated. If such a solution is set aside to cool, the mouth of the flask being covered with a sheet of paper to keep out dust, &c., it will not deposit crystals as an ordinary solution would. The reason of this is that the solution is in a state of strain, each particle of the sulphate of soda trying to unite with those around it to form crystals, and attracting and being equally attracted by all. These forces balance one another, and the liquid is in a state of equilibrium, which may last for hours, and even days. But should the equilibrium be accidentally disturbed by shaking the flask, or introducing a particle of foreign matter or a crystal of sulphate of soda, crystallisation will im- mediately take place. Stars and spikes of sulphate of soda will rapidly form in the liquid, and _grow and intertwine with one another, forming ice-like fronds. The process, though slow at first, will proceed with bewildering rapidity, until in less than a minute nearly the whole of the liquid has become
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