On the Theory of Manures. ' 15 



and multiply and get matted around them, whlcli of itself must 

 greatly assist. Whether the roots are impatient of too much 

 moisture, or attracted by the gases absorbed as they cling 

 round those charred pieces of wood, and of clay as in pieces of 

 pot and brick, or whether opposite states of electricity may 

 affect, it might be difficult to say. It is likely, however, that 

 the absence of undue moisture, and the presence of a proper 

 quantity of gases, form the principal cause. I have seen them 

 cling round large pieces of porous bones with avidity, while 

 they refused to enter the powdered small pieces of bones, where, 

 perhaps, there was too great an abundance of food, the ends 

 of the spongioles appearing diseased and swoln. The carbon 

 of the ashes is also undoubtedly given off as the pieces begin to 

 dissolve. I have seen such powder to increase greatly to 

 appearance the growth of hyacinths in water, as compared with 

 those in pure water ; and the saline substances and carbon were 

 likely to afford most of the benefit there. 



To enable us to arrive at correct data, plants should be ana- 

 lysed in the various stages of their existence. The comparative 

 analyses of plants must vary according to the age of the plant 

 experimented on ; according to the part of the plant examined 

 also, if in portions ; perhaps, also, according to the liberality of 

 the supply furnished to the roots. What is to be allowed for 

 constituents, and what for assistants, and what perhaps deducted 

 for superfluity, may be very difficult to solve, and may require 

 many analyses of scientific men, and much judicious observation 

 of practical men, before arriving at correct principles. Super- 

 fluity is undoubtedly prejudicial ; and it has been frequently 

 proved by experience, that plants will at times absorb both 

 more water and more food in the water than are necessary, or 

 than the leaves can elaborate, so as to enable the organs to as- 

 similate. As the quantity of food absorbed depends partly on 

 the quantity of water absorbed, excess in some seasons and 

 some soils may more readily occur than in others. In seasons 

 when little light is present, less water is, however, absorbed, 

 though the season should be more wet and the ground more 

 moist ; and, to a certain extent, the one is a corrective of the 

 other : yet confusion inay arise from the confliction of causes, 

 and may baffle and retard the efforts at establishing rules ; 

 though, if we are cautious of deceiving ourselves and others 

 with preconceived notions, and do not attempt to make practice 

 bend to theory, it will undoubtedly in the end conduct us to 

 the right path. 



In endeavouring to ascertain from practice the necessity for 

 these substances, by the effects produced by their application, 

 the recorded effects of experiments are much at variance. 

 Nitrate of soda has been found to have very different effects in dif- 



