356 On Paring and Burning. 



detail, the nature of the physical and chemical chansjes which 

 take place in clay soils on burning. In order to save tlie general 

 reader the trouble of reference, 1 will just biiefly state some of 

 the results which I obtained in a number of analytical experi- 

 ments, and beg to refer those specially interested in clay-burning 

 to Vol. XII. page 496, of this Journal, 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 clav upon burning are highly important, they do not 

 sufficiently explain the full benefits which are obtained on the 

 application of Ijurnt 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 cla\', 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 propqrtion 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 ineihcacy of overburnt 

 <lay is due partly to the mechanical changes which clay undergoes 

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

 consequence of its diminished porosity, becomes less efKcient 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 : — 



" Deak Sir, — T now send j'ou a specimen of clay, which underlies a hlackisli 

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

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

 desirous of ascertaiuins;, first, Avhether it is of a description ■which would pay 

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

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

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

 from which this clay comes hes against the railway, and is quite of a diflfercnt 



