C sia ] 



:o the diminution of the coherence and tenacity of 

 clays, and to the destruction of inert, and useless ve- 

 getable matter, and its conversion into a manure. 



Dr. Darwin, in his Phytologia, has supposed, 

 that clay during torrefaction, may absorb some nutri- 

 tive principles from the atmosphere that afterwards 

 may be supplied to plants ; but the earths are pure 

 metallic oxides, saturated with oxygene ; and the ten- 

 dency of burning is to expel any other volatile princi- 

 ples that they may contain in combination. If the 

 oxide of iron in soils is not saturated with oxygene, 

 torrefaction tends to produce its further union with 

 this principle ; and hence in burning, the colour of 

 clays changes to red. The oxide of iron containing 

 its full proportion of oxygene has less attraction for 

 acids than the other oxide, and is consequently less 

 likely to be dissolved by any fluid acids in the soil ; 

 and it appears in this state to act in the same manner 

 as the earths, A very ingenious author, whom I 

 quoted at the end of the last Lecture, supposes that the 

 oxide of iron when combined with carbonic acid is 

 poisonous to plants ; and that one use of torrefaction 

 is to expel the carbonic acid from it ; but the carbon- 

 ate of iron is not soluble in water, and is a very inert 

 .substance ; and I have raised a luxuriant crop of cres- 

 ses in a soil composed cf one-fifth carbonate of iron, 

 and four-fifths carbonate of lime. Carbonate of iron 

 abounds in some of the most fertile soils in England* 

 particularly the red hop soil. And there is no theo- 

 retical ground for supposing, that carbonic acid, which 

 Is an essential food of plants, should in any of its com- 



