70 THE SOIL. 



insoluble and the lime soluble. There is evidently here 

 some other attractive force at work, which alters the 

 effect of chemical affinity. If a solution of phosphate of 

 lime in water containing carbonic acid is filtered tlirough 

 a funnel filled with earth, the uppermost layer of the 

 earth first takes up the phosphoric acid or the phosphate 

 of hme from the fluid. Once saturated therewith it no 

 longer stops the free passage of the dissolved phosphate 

 of lime which now reaches the layer beneath ; the latter 

 then again becomes saturated in the same way, and thus 

 by degrees the phosphate of hme is Completely diffused 

 throughout the earth in the funnel, so that every particle 

 retains on its surface an equal proportion of this substance. 

 If the phosphate of hme were of the colour of madder and 

 the soil colourless, the latter would now actually present 

 the appearance of a madder lake. Just in the same way 

 potash is diffused through the soil when a solution of 

 carbonate of potash is filtered through it ; the lower 

 layers receive what the upper do not retain. 



There is no need of any special disquisition to show 

 that the phosphate of hme contained in a particle of bone- 

 earth is diffused in exactly the same way through arable 

 soil, with this difference, that the solution of phosphate of 

 lime in rain-water containing carbonic acid is effected at 

 the very spot where the particle hes, and spreads thence 

 downward and in all directions. 



The potash and the silicic acid rendered soluble by 

 dismtegration, or by the action of water and carbonic acid 

 upon silicates, are diffused through the soil in the same 

 way, so is ammonia also, which is conveyed m rain-water, 

 or is generated by the putrefaction of the azotised consti- 

 tuents in the decayed roots from the successive generation 

 of plants grown on a field. 



Every soil must therefore contain potash, silicic acid 



