110 THE SOIL. 



bination partly with carbonic acid, partly with mineral 

 acids ; and there can hardly be a doubt but that the 

 plant absorbs a portion of these substances from the 

 solution. The same must apply equally to potash, 

 ammonia, and the dissolved phosphates ; but the water 

 circulating in the soil, in a normal condition, either does 

 not hold the three last-named substances in solution, or 

 not in sufficient quantities to supply the demands of 

 the plant. 



According to the ordinary rules of natural science, 

 when we seek to explain a phenomenon, we leave out 

 of view those cases in which the conditions superinduc- 

 ing the phenomenon are clear and patent. For in- 

 stance, if we find in bog-water all the ash-constituents 

 of duckweed, there can be no doubt about the form in 

 which they passed into the plant ; they were dissolved 

 in water, and they were absorbed in a soluble state. 

 In such a case, we have merely to explain the reason 

 why the several ash-constituents, being all present in 

 one and the same form, have yet passed into the plant 

 in unequal proportions. 



If, in another case, W T C find that the rain-water 

 which falls on a given area of land, dissolves out of the 

 soil many times more potash than was contained in a 

 crop of turnips grown on that area, there is every rea- 

 son to assume that the turnip, like the duckweed, has 

 absorbed the needful potash from a solution. But, if 

 in the entire quantity of water which falls on the field 

 during the period of vegetation, we find only just so 

 much potash as the turnip crop requires, and no more, 

 the assumption that the potash in the turnips has been 

 derived from this solution would necessarily involve 

 the impossible supposition, that all the watery particles 

 containing potash must have been in contact with the 

 roots of the turnips ; otherwise, the latter could not 

 have absorbed so much potash as is actually found in 

 them. This supposition is impossible ; because, during 

 the time when the turnip vegetates, there is generally 

 no water circulating in the soil such, for instance, as 

 might be carried oft by drain-pipes. 



