19 



The usual method consists in fusing the finely powdered sub- 

 stance 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 combina- 

 tion with potash and soda : silicate of baryta is formed, and the 

 alkalies, potash and soda, uniting with the carbonic acid of the 

 carbonate of baryta, are rendered soluble. 



Lime, which in its chemical relation is closely allied to baryta, 

 acts precisely in the same manner on insoluble silicates of potash 

 and soda. Now, if clay originally contains carbonate of lime, it 

 will act at an elevated temperature on the insoluble silicate of 

 potash, which is present in many clays in the shape of fragments 

 of felspar ; and by duple decomposition it will give rise to the 

 production of silicate of lime and carbonate of potash. Silica 

 enters into combination with lirne 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 acid. The 

 diminution of the quantity of lime, and the increase of potash in 

 the soluble portion of burnt clay, thus find a ready explanation. 

 Much, however, as indicated by the practical observations, and the 

 above analytical results, depends on a proper temperature. If the 

 heat is allowed to become too intense, new changes in the con- 

 stituents of clay are produced, which have the effect of rendering 

 the potash again less soluble. 



The fact that felspar is more readily decomposed after having 

 been moderately calcined is not a new one, Professor Fuchs of 

 Munich having shown clearly that this is the case, not only with 

 felspar, but also with other minerals, into the composition of which 

 silicate of potash enters. Fully in accordance with this fact is the 

 practical observation of Professor Lampadius, who found, by a 

 series of field experiments, that moderately calcined gneiss, granite, 

 certain kinds of porphyry and trap rocks, all of which contain 

 silicate of potash, in a similar manner as burnt clay, promote the 

 luxuriant growth of many plants in a remarkable manner. It 

 would be doing injustice to Professor Zierl of Munich to leave 

 unnoticed that, in speaking of the causes of the effects of burnt 

 clay, he suggested whether the accessary constituents of clay, 

 particularly the alkalies, might not be rendered more soluble in the 

 process of burning. Had Professor Zierl submitted unburnt and 

 burnt clay to chemical analysis, he would, no doubt, have found 

 that this was really the case ; but as he brought forward no 

 experimental proof in support of his theory, it had the fate of 

 being disregarded by many at the time of its publication, and of 

 being soon after forgotten by most. 



As far as I am aware, the above analytical results, in support of 



