On the Causes of the Efficacy of Burnt Clay. 



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the constituents of felspar, that on these grounds he recommended 

 a process of extracting- and manufacturing; potash on a large scale 

 from felspar. He has shown that, under these circumstances, 

 insoluble silicate of iime and soluble carbonate of potash are 

 formed. 



3. By ichat qualities or characteristics are clays totally unfit for 

 burning distinguished from those which jirove most efficacious 

 after burning ? 



Pure clays, such as pipe and porcelain clay, consist almost 

 entirely of silica and alumina; and as silicate of alumina does not 

 itself serve as direct food to plants, they are found to be sterile in 

 their natural state, and will remain so after burning. We have 

 seen that the accessary constituents of clays furnish the materials 

 from which plants derive their inorganic food, and that potash is 

 the most valuable constituent of clays. The proportion of pot- 

 ash in clays varies considerably; whilst some contain from 2 to 4 

 per cent, of potash, others contain but mere traces. Now, if 

 it be true what has been advanced with respect to the causes 

 of the efficacy of burnt clay, I have no hesitation in pronouncing 

 all clays which contain no potash at all, or mere traces, as 

 totally unfit for burning. Experience, I think, will prove that 

 all clays which nearest resemble in composition pipe and. 

 porcelain clays, and are naturally very infertile, will not in the 

 least be improved by burning. On the other hand, the more un- 

 decomposed potash in the shape of felspar or any other mineral 

 a clav contains, the more useful it will be found after burninGr. In 

 support of the theory I have embraced, with respect to the efficacy 

 of burnt clay, it may be mentioned that I have determined the whole 

 amount of alkalies which the clay from Huntstile farm contained 

 in any form. The greater part of potash and soda in this clay is 

 present in the state of insoluble silicate ; and as I find no less 

 than 4726 per cent, of potash and '88 per cent, of soda, I have 

 no hesitation in suggesting that this clay is most likely to prove very 

 efficacious after burning. With this theoretical speculation agrees 

 well the fact mentioned by Mr. Danger, the tenant of Huntstile 

 farm, that by burning this clay the land is very much improved. 

 Mr. Danger says : — Of course I can only speak to the fact. A 

 soil, which 1 have found quite sterile, on which this process has 

 been used, became totally changed.''^ 



4. Can it be determined by Chemical Analysis, xchether burning 



will he efficacious in rendering Clay a fertiliser ; or not 2 

 From the preceding remarks it follows that the fertilizing- 

 effects of clay mainly depend on the proportion of potash which 

 it contains ; and as any good analytical chemist may determine the 



