76 FARM DEVELOPMENT 



room. The soil in the pot will again increase in weight, 

 since it will be able to gather more water from this very 

 moist air than it had secured from the relatively dry air 

 which came through the windows. We will assume that 

 the one hundred pounds of fire-dried soil, when exposed 

 for some days to the outside air, absorbed three pounds 

 of water, and that when it was put in the air made very 

 moist by the boiling water, it absorbed two pounds more. 

 This moisture is held in the soil as a delicate film about 

 each soil particle, or within the minutest pores in the 

 particles of soil, but is not sufficient to bind the particles 

 together as does the larger amounts of capillary moisture 

 mentioned below. The finest, driest road dust con- 

 tains from one to ten per cent of moisture in this 

 form. 



By means of a fountain throwing a very fine spray on 

 the surface of the soil in the pot, a miniature rainstorm 

 may now be produced in the room. As the tiny rain- 

 drops strike the surface of the air-dry soil, they are 

 eagerly seized by the surfaces of the small particles of 

 soil. While the soil could not gather and condense any 

 more of the vapor from the air and associate it with its 

 own particles, the surfaces of soil particles at once show 

 a strong attraction for this finely sprayed water, or vapor 

 condensed to the liquid form. The water and the sur- 

 faces of the soil particles seem to desire the closest 

 touch with each other, and, as the water is a mobile fluid, 

 it spreads out in thin layers over the attracting surfaces 

 of the minute soil particles, enters into the pores within 

 the particles, and fills the capillary spaces among them. 

 If one particle has a thicker layer of -water on its sur- 

 face than its neighbor, the water is soon equalized over 

 all. If in traveling from particle to particle the film of 

 water finds a pore space or interstice unoccupied, it flows 

 into that. As the rain proceeds the particles at the top 

 of the soil become surcharged with water. The water- 



