face of the earth is dependent upon exactly the same conditions as the formation of dew. It used to be thought that, as soon as the air was cooled to or below the dew point, the molecules of water vapor in the air would come together and form drops of water. In 1880 Mr. John Aitken, of Scotland, began a long and very thorough series of experiments upon the condensation of water vapor f ron? the air, and the same line of experimentation has been carried still further by Robert von Helmholtz and by Richarz in Germany. These experiments have all shown that vapor condensation within the body of the air only takes place upon the surface of dust particles which are floating in the air. Indeed, Robert von Helmholtz found that when the air was carefully freed from dust particles it could be cooled until it contained ten times the amount of vapor necessary to saturate it without any condensation taking place within the body of the air. Aitken thought that he had found one exception to this, and that in the case of a sudden shock upon the walls of the containing vessel, when the air within was oversaturated, precipitation would take place; but Robert von Helmholtz found that this apparent exception was due to the dust particles given off by the walls of the vessel at the time of the shock. Since this fact has been experimentally established, Lord Kelvin has shown mathematically that, from the known laws of surface tension in water, it would be impossible for a globe of water consisting of only a small number of molecules to hold together at all. The same calculation has been made by Robert von Helmholtz by means of a formula developed by his illustrious father. According to these calculations, the smallest sphere of water which could hold together at 0º C. would be ·00015 millimetre or ·000006 inch in diameter. Since this is 7,500 times the diameter of a water molecule as computed by Lord Kelvin, the smallest drop of water which could be held together by cohesion at this temperature would contain not less than four million millions of water molecules. At 40º the smallest possible water sphere would have a diameter about twice as great, and would accordingly contain eight times as many molecules.
Aitken found that dust particles of microscopic size were sufficient for the nuclei of condensation, and R. von Helmholtz showed that condensation could take place upon particles so small that it took four days for them all to settle through still air to the lower side of a horizontal glass tube about one inch in diameter.
Aitken counted the number of these dust particles in different samples of air by first diluting the air with two hundred times its volume of air which had had its dust particles removed by being drawn through water, and then saturating the air with water and cooling far below its dew point, and counting the number of
