142 A PRAC riCAL TREATISE 



but clay, of all sorts, and strong loams, are well adapted to the 

 purpose. It has, however, been observed by Mr, Burroughs, 

 'that this mode of preparmg earth, as a manure, is upon a 

 principle very different from that of reducing it to aslies; and 

 thougi) the effect produced on the land to which it is applied 

 may be apparently the same, yet the torrified' (dried by fire, 

 or scorched) 'substance, is by far more permanent. The 

 chemical difference in their preparation is this : — In reducing 

 tlie soil to ashes, much of its fertilizing properties are dissi- 

 pated, and its vegetable matter destroyed; but, when only 

 torrified, those properties are preserved, and its vegetable mat- 

 ter only reduced to a state more valuable as the food of plants.' 



'There are two sorts of burned earth, — 1st, that which is 

 obtained from the surface, of which we have already treated; 

 and, 2ndly, that which is dug from the sub-soil.' 'Sub-soil, 

 calculated for this purpose, may be said to be of two kinds, viz., 

 adhesive clay, and calcareous earth: the former seldom pos- 

 sessing any fertilizing properties in its natural state; but the 

 latter is, even in that state, generally a valuable substance. 

 As alteratives to the soil, tliey may be good applications, if 

 appropriately applied, and may produce fertility accordingly; 

 but by putthig them through a process of torrefaction, how- 

 ever naturally deficient in fertilizing properties, they can be 

 converted into valuable manures,' 



Mr. Naismith also says, 'that when cohesive earth has 

 undergone this operation, and is afterwards reduced to powder, 

 all its tendency to coherence is lost, and its particles lie com- 

 pact, without uniting. It seems to have the perfect consist- 

 ence of a fertile soil; for corn springs quickly, and tillers 

 abundantly on it; and if a little dung-juice be given from time 

 to titne, it will grow luxuriantly to maturity. It has also a 

 strong tendency to promote fertility when applied to other 

 soils ; and even a very slight torrefaction of the soil has a 

 great effect. 



Effects of burnt clay. — From this we should be led to con- 

 clude that a very material difference would be found to exist 

 in earth thus merely dried; for there can be no doubt that its 

 properties must depend upon the quantity of vegetative matter 

 which it contains. But it is difficult to draw the line between 

 this process of torrefaction and that which consists in burning 

 the earth to ashes: for if the clay be merely dried, the next 

 rains will reduce it back to its former state, by which the soil 

 cannot be benefited ; and if the fire be advanced to a degree 



