66 THE PHYSICAL CONDITION OF A SUBSTANCE, 



5. Influence of the physical condition of a substance, 'its state 

 of chemical combination, and its tendency to decompose in a 

 given soil, on the immediate effects it produces. 



1. Physical condition. In the preceding section, I have ex- 

 plained the advantage of extreme mechanical division in exalt- 

 ing the immediate action of fertilising substances. But, even 

 when minutely divided, there are circumstances in which a 

 peculiarity in the physical or molecular condition may modify 

 the apparent action of two different samples of the same sub- 

 stance similarly applied. Thus, 



a Gypsum, when burned, parts with its water and becomes 

 friable. It is then easily reduced to a fine powder, and, when 

 applied to the soil, can be easily spread and intimately mixed. 

 But if in the burning it be heated above 300 F. its physical con- 

 dition is altered ; and though it is still easily reduced to powder, 

 it ceases to harden when mixed with water, as common stucco 

 does, and becomes nearly insoluble in water. No experiments 

 have yet been made to ascertain the comparative action of this 

 overburned gypsum upon the land ; but if it act sensibly at all, 

 we can scarcely expect it to act so speedily as native or ordi- 

 narily burned gypsum is found to do. 



b Again, some varieties of clay are found to be so changed 

 by burning, as greatly to benefit the soil upon which they are 

 laid. But if such clays are overburned heated too long, and 

 to too high a degree they cease to possess this improving 

 quality, or possess it in a less degree. Like the gypsum, the too 

 great heating has rendered them less soluble than before, and 

 thus less capable of yielding to the soil and plant the ingredi- 

 ents they contain. 



Such physical differences may exist in the case of many other 

 substances which may be used for agricultural experiments and 

 the criticiser of results ought to bear this in mind, as affording 

 a means of explaining discordances among field trials which 

 might otherwise appear inexplicable. 



2. State of chemical combination. Suppose two substances 

 we apply two sulphates, for example to act solely in con- 

 sequence of the sulphuric acid they contain, that which was 



