500 



On the Causes of the Efficacy of Burnt Clay. 



formation of ammonia cease. I have examined burnt clays 

 which contained no protoxide of iron at all, and which^, neverthe- 

 less, were found most efficacious as a manure. This fact alone 

 contradicts Sprengel's theory ; but in order to learn if in cla}^, 

 containing much protoxide of iron, more ammonia is formed than 

 is absorbed by the same clay from the atmosphere, I made the 

 following experiments ; — 



A portion of clay, from Huntstile, was moderately burnt in a 

 closed crucible, after having been previously mixed with 1 per 

 cent, of charcoal powder. The charcoal powder was mixed with 

 the clay for the purpose of reducing the peroxide of iron in the 

 clay to protoxide. 



a. One-half of the clay thus treated was exposed for two 

 months and fourteen days to a dry atmosphere, in a dry state : — 



182 '81 grains gave 0*28 grains of bichloride of platinum and am- 

 monium ; or, 



100 parts gave 0* 17 per cent, of ammonia. 



h. The other half w^as thoroughly moistened with water, and 

 exposed for the same length of time to the atmosphere : — 



212-11 grains gave 0*33 grains of bichloride of platinum and am- 

 monium ; or, 



100 parts gave 0*18 per cent, of ammonia. 



These quantities of ammonia are nearly identical. Ammonia, 

 accordingly, is not formed, as Sprengel supposes, by the decom- 

 position of water under the influence of protoxide of iron and 

 the atmosphere, in a larger quantity than is absorbed by dry 

 clay from the atmosphere. 



Thus, under no circumstances, do we find ammonia in burnt 

 clay in so large a quantity as to justify us in regarding it as the 

 chief cause of efficacy in burnt clay. The effects of burnt clay 

 cannot be explained by the absorption of ammonia only, nor, 

 indeed, by the mechanical changes which burnt clay has under- 

 gone after burning. In a great measure, the efficacy of burnt 

 clay is due to the chemical changes to which the constituents of 

 clays are subject in burning. 



I shall therefore consider : — 



II. The Chemical Effects of Heat on Clay.— In an excellent 

 paper on the effects of burnt clay, Professor Johnston expresses 

 the opinion that the mechanical e^^ects of burning upon clay are 

 insufficient to explain the efficacy of burnt clay ; and proves 

 experimentally that clay, upon burning, is subject to chemical 

 changes, which are of such a kind as to render the constituents 

 of clay more soluble. To these chemical changes the learned 

 Professor attaches even more importance than to the mechanical 

 alteration of the condition of the soil. 



