§ 107. THE PHYSICAL QUALITIES OF THE SOIL. 195 



appear. For a standard of comparison it would be well 

 to carry on, simultaneously, one or two similar trials with 

 soils of a marked character, such as a very strong clay 

 soil and a very sandy one, that have been tested before 

 in this respect. 



To make the determination, use the zinc box filled with 

 wet soil, that was obtained in testing the water-holding 

 power ; put each box with its contents in a second box 

 of thick j^asteboard, into which it just fits, and then put 

 all these pasteboard boxes close together in a third 

 wooden box, just as deep as the zinc boxes; provide the 

 wooden box with a cover, in which holes are so cut that, 

 when the cover is on the box, only the surface of earth in 

 each zinc box is exposed. 



Put the whole where the sun's rays can fall on the soil, 

 and weigh each zinc box with its contents every two or 

 three days, and during a length of time varying from. two 

 to four weeks, according to the weather ; frequent ob- 

 servations of the temperature and the state of the sky 

 should be made, while the evaporation is going on. 



It will be observed that, in the beginning, the rate of 

 evaporation is about the same for all the varieties of soil 

 under examination, even when exposed to the rays of a 

 hot sun ; after a time the sandy soils begin to lose weight 

 more rapidly than those in which clay or humus pre- 

 dominates ; the difference increases up to a certain point, 

 and then begins to diminish, until, after a time, the rate 

 of evaporation is nearly the same again for all ; this con- 

 tinues for a time, and then the clay and humus soils begin 

 to lose water more rapidly than the light loam, because 

 the latter is, by this time, nearly air-dry. 



It is of course more important to watch carefully the 

 rate of evaporation, from the time when it begins to differ 

 in the different soils, to the time when it again becomes 

 about the same for all. 



