AGRICULTURAL CHEMISTRY. 155 



is done by green manuring. The table gives ns proof that gypsum 

 does nol exert any beneficial action in consequence of directly attract- 

 ing moisture. Humus, or decaying vegetable matter, it will be seen, 

 surpasses every other ingredient of the soil in absorbing 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 predominate so 

 dry and hot that vegetation perishes from want of moisture; when, 

 however, they occur as fine dust, they form too wet a soil, in which 

 plants perish from the opposite cause." — (Hamm's Landwirthschaft.) 

 In the fact that soils have a physical absorbing power for the vapor 

 of water, we have an illustration of a general principle, viz: That the 

 surfaces of liquid and solid matter attract the particles of other kinds of 

 matter. In the same way that water is absorbed, oxygen gas is con- 

 densed, especially in certain highly porous bodies. Platinum, copper, 

 lead, and iron, when in the state of fine sponge, exert a remarkable 

 condensing power on oxygen, and it is probable that thereby this 

 element is ozonized. Platinum sponge exhibits the characters of a 

 body charged with ozone, and it is to be anticipated that investiga- 

 tion will shortly demonstrate the occurrence of gaseous condensa- 

 tions in the soil, the effect of which is to produce chemical changes 

 of the most important character. It is not unlikely that the organic 

 matters of the soil, which possess the extremest porosity may thereby 

 acquire their power of ozonizing the oxygen which combines so 

 readily with them, and thus accomplish the formation of nitric acid 

 from atmospheric nitrogen. 



Of exceeding influence on the fertility of the soil is, thirdly, its 

 permeability to liquid water. 



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 hnvefeiv 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 attraction 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 one-half 

 inch in diameter, the surface of the liquid will be curved or concave. 

 In a very narrow tube the liquid will rise to a considerable height. 

 In these cases the surface attraction of the glass for the water neu- 

 tralizes or overcomes the weight of (earth' s attraction for) the latter. 

 The pores of a sponge raise and hold water in them, in the same 



