INCREASE OF CAPILLARY ATTRACTION. 133 



lower state of oxidation. Thus it converts the red, 

 ov 2)ei'-oxide of iron, into the j;ro/-oxide, and acts in 

 the same manner upon the oxides of manganese. 

 It also takes their oxygen from the sulphates (as 

 from gypsum), and converts them into sulphurets. 

 These lower oxides of iron and manganese are in- 

 jurious to vegetation ; and it is one of the beneficial 

 purposes served by turning up the soil in ploughing, 

 or by otherwise loosening it, so as to allow the free 

 admission of the atmosphere, that the natural pro- 

 duction of these oxides is either in a great measure 

 prevented, or, when produced, they speedily become 

 harmless by the absorption of an additional dose of 

 oxygen." 



5. Drought has an injurious effect on unpulver- 

 ized land. 



The pulverization of the soil increases capillary 

 attraction, and therefore, to a great extent, prevents 

 drought. Prof Leslie found, upon investigation, 

 that capillary attraction increased in an inverse 

 ratio of the diameter of the soil : if the spaces were 

 one hundredth of an inch, the attraction would 

 extend about four inches, but w^ith those of one ten 

 thousandth, it was increased to twenty-five feet. 



The illustration of the effect of pulverization 

 upon the soil in preventing drought is somewhat 

 similar, as is the result, to that under the same 

 head in the chapter upon draining, (a) in the figure 

 represents the soil which is tilled of only a few 

 12 



