106 ANNUAL REPORT OF NEW-YORK 



in most cases, was due to this combination of it with the elements 

 of humus, or with the protoxyd of iron, the latter passing thereby 

 into the peroxyd; but in case of magnesia and carbonate of lime 

 the absorption must have been a surface condensation. 



An obvious practical result follows from the facts expressed in 

 the above table, viz : that sandy soils which have little attractive 

 force for watery vapor, and are therefore dry and arid, may be 

 meliorated in this respect, by admixture with clay, or better with 

 humus, as is done by green manuring. The table gives us proof 

 that gypsum does not exert any beneficial action in consequence 

 of attracting moisture. Humus, or decaying vegetable matter, it 

 will be seen, surpasses every other ingredient of the soil in absor- 

 bing moisture. This is doubtless in some degree connected with 

 its extraordinary porosity or amount of surface. How the extent 

 of surface alone may act, is made evident by comparing the 

 absorbent power of carbonate of lime, in the two states of sand, 

 and of an impalpable powder. The latter it is seen, absorbed 

 twelve times as much vapor of water as the former. Carbonate 

 of magnesia stands next to humus, and it is worthy of note that 

 it is a very light and fine powder. 



Finally, it is a matter of observation that " silica and lime in 

 the form of coarse sand, make the soil in which they predomi- 

 nate so dry and hot that vegetation perishes from want of mois- 

 ture; when, however, they occur as fine dust, they form too wet 

 a soil, in which plants perish from the opposite cause." {^Hamrn's 

 Lan dwirths c haft . 



in. Permeability of the soil to water^ including percolation and 

 capillarity. — A soil is permeable to water when it allows that 

 liquid to soak into or run through it. To be permeable is of 

 course to be porous. On the size of the pores depends its degree 

 of permeability. Coarse sands and soils which have few but 

 large pores or interspaces, allow water to run through them 

 readily — water percolates them. When instead of running 

 through, the water is largely absorbed and held by the soil, the 

 latter is said to possess great capillary power; such a soil has 

 many and minute pores. The cause of capillarity is the same 

 surface j^ttraction which has been already mentioned. 



When a narrow vial is partly filled with water, it will be seen 

 that the liquid adheres to its sides, and if it be not more than 



