RAIN-MAKING. 483 



face of tlie earth is dependent upon exactly tlie 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 care- 

 fully^ freed from dust particles it could be cooled until it con- 

 tained 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 exceyjtion to this, and 

 that in the case of a sudden shock upon the walls of the contain- 

 ing 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 mathe- 

 matically 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 C. would be '00015 millimetre or -000006 inch in 

 diameter. Since this is 7,500 times the diameter of a water mole- 

 cule 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 mole- 

 cules. At 40 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 



