18 On Paring and Burning. 



rather more than double that quantity, or, in exact numbers, '11 

 per cent. 



That burning has the effect of rendering other constituents 

 besides potash more soluble in dilute acids, is clearly shown in 

 the subjoined analytical results obtained with this clay in its 

 natural state, and when burnt, at various temperatures. In two 

 separate determinations it gave in its natural state : 



1st Experiment. 2nd Experiment. 



Insoluble matters in dilute acids .. 56*30 55'17 



Soluble .. 43-70 44'83 



containing potash (-35) 



100-00 100-00 



Heated very moderately it yielded : 



Insoluble matters in dilute acid .. 50'90 52-31 



Soluble .. 49-10 47'69 



containing potash ('77) 



100-00 100-00 



Exposed to a somewhat stronger, but still by no means a very 

 high temperature, it produced : 



Insoluble matters in dilute acids 46*20 



Soluble matters 53'80 



100-00 



These experiments thus show plainly that the solubility of clay 

 in dilute acids is regulated by the degree of heat to which it is 

 exposed. A certain degree of heat is necessary to induce a proper 

 chemical reaction ; but, as demonstrated by former experiments, 

 an excessive heat should be carefully avoided, inasmuch as it has 

 the effect of rendering burnt clay again less soluble. It is, no 

 doubt, for this reason, and not on account of a little charcoal which 

 is obtained, that practical men recommend stifle burning, for this 

 prevents the heat of the heaps of burning clay getting too intense, 

 and consequently stifle-burnt clay is always very porous, crumbles 

 readily to powder, and is more easily soluble than clay burnt at too 

 high a temperature. In accounting for the advantages of paring 

 and burning, the changes in the inorganic matters of the soil must 

 not be overlooked ; for soils which are regularly pared and burned 

 often contain much organic matter, and afford therein fuel for 

 burning a considerable quantity of the soil itself. 



ON THE SOILS WELL ADAPTED FOR PARING AND BURNING. 



As stated already, it is not all soils that can be pared and 

 burned with advantage. It becomes, therefore, a matter of some 



