16 On Paring and Burning. 



and chemical changes which take place in clay soils on burn- 

 ing. In order to save the general reader the trouble of reference, 

 I will just briefly state some of the results which I obtained 

 in a number of analytical experiments, and beg to refer those 

 specially interested in clay-burning to Vol. XII., page 496, of the 

 Journal of the Royal Agricultural Society, where a paper of 

 mine will be found, entitled, On the Causes of the Efficacy of 

 Burnt Clay/ 



The mechanical effects of heat on clay are simple and easy to 

 be understood. Heavy stiff clay soils are impervious to water, 

 very tenacious and unctuous, and for these reasons often cold and 

 expensive to work. Burning alters, at least to some extent, these 

 undesirable qualities, and tends to render naturally stiff soils more 

 porous and friable. But although the mechanical effects pro- 

 duced on clay upon burning are highly important, they do not 

 sufficiently explain the full benefits which are obtained on the 

 application of burnt clay or soil to the land. By a series of 

 experiments I showed that in burning clay soils effects similar to 

 those of fallowing are produced ; and that many of the con- 

 stituents of clay, more especially potash, are rendered more 

 soluble by burning. It is to the potash liberated from clays 

 on burning that I am inclined to ascribe the chief benefits 

 resulting from the application of burnt soil as a manure. I 

 further showed that those clay- soils, which contain originally 

 a large proportion of undecomposed silicates of potash and soda, 

 are best suited for burning, whereas soils and clays, resembling 

 in composition pure pipe or porcelain clays, and all soils which 

 contain mere traces of undecomposed alkaline silicates, are unfit 

 for burning. Finally, I showed that the inefficacy of overburnt 

 clay is due partly to the mechanical changes which clay undergoes 

 in everburning, whereby it is rendered hard like stone, and, in 

 consequence of its diminished porosity, becomes less efficient as 

 an absorber of ammonia ; partly to the chemical changes, whereby 

 the constituents of clay soils are rendered less soluble than they 

 are even in their natural state. Subsequent analyses of clays in 

 their natural condition, and after burning, have fully confirmed 

 the above-mentioned general results. Amongst others, I may 

 instance a clay which has been sent to me for examination by 

 Charles Lawrence, Esq., accompanied by the following note, which 

 I have no doubt will be read with some degree of interest : 



" DEAR SIR, I now send you a specimen of clay, which underlies a blackish 

 loam of variable thickness, from twelve to eighteen inches and more in depth ; 

 and, in reference to an article of yours which I read some time ago, I am 

 desirous of ascertaining, first, whether it is of a description which would pay 

 for burning and spreading over the surface, and, secondly, whether it contains 

 constituents which would render it eligible as a manure to spread over and get 

 it incorporated with light land which gets into a dust in the spring. The field 

 from which this clay comes lies against the railway, and is quite of a different 



