504 



0;i the Causes of the Efficacij of Burnt Clay. 



clay in dilute muriatic acid, it is clear that the lime did not exist 

 in a state of carbonate. The lime must have existed in Nos. II ., 

 III., and IV. as caustic lime, or in a state of silicate. 



The excess in analyses Nos. II., III., and IV. is partly due to 

 this inaccuracy of stating the results, partly to the fact that silicate 

 of protoxide of iron in burninof becomes decomposed. The prot- 

 oxide of iron is rendered soluble in dilute muriatic acid, but in 

 the analyses it is obtained and calculated as peroxide ; hence we 

 find the largest excess in No. 11. , in \Yhich most iron has become 

 soluble in dilute acid after burning. 



The following considerations, which at the same tim.e explain 

 the mode in which potash is liberated from the constituents of 

 clay and rendered soluble, induce me to think that the lime in 

 Nos. IL, III., and IV. existed in a state of silicate. The usual 

 method of determining the quantity of potash and soda in inso- 

 luble silicates consists in fusing the finely-powdered substance 

 with an excess of carbonate of baryta. In this process potash and 

 soda are rendered soluble in the following manner : the baryta 

 combines with the silica, originally present in combination with 

 potash and soda ; silicate of baryta is formed, and the alkalies 

 potash and soda uniting vvith the carbonic acid of the carbonate 

 of baryta are rendered soluble. 



Lime, which in its chemical relation is closelv allied to barvta, 

 acts precisely in the same manner on insoluble silicates of potash 

 and soda. If clay originally contains carbonate of lime, it will 

 act at an elevated temperature on the insoluble silicates of potash 

 and soda, present in many clays in the form of fragments of 

 felspar or mica, and by duple decomposition it will give rise to 

 the production of silicate of lime and carbonate of potash. 

 Silica enters into combination with lime in different proportions ; 

 some of these combinations are soluble in dilute acids, most of 

 them are insoluble. Instead of carbonate of lime and insoluble 

 silicate of potash, we thus find in burnt clay a larger proportion 

 of soluble potash and silicate of lime, which is partly insoluble in 

 dilute acids. The diminution of the quantity of lime, and the 

 more abundant proportion of potash in the soluble part of burnt 

 clay, thus find a ready explanation. Much, however, as indi- 

 cated by the practical observations and the above analytical 

 results, depends on the proper regulation of the temperature. 



2. How does Burnt Clay act in improviny the Soils ; or, in other 

 icords, what are the causes of the efficacy of Burnt Clay ? 



The answer to this question will become much more intelli- 

 giblcj after we shall have considered briefly the origin and com- 



