18 



muriatic acid in a remarkable manner. In the natural clay only 

 0.269 of a per cent of potash were soluble ; whereas in clay burnt 

 at a moderate heat, and under circumstances resembling those 

 under which clay is burnt in the field, the quantity of soluble 

 potash amounted to more than three times the former quantity, the 

 exact proportion of potash being 0.941 of a per cent. In clay 

 No. 3, the higher temperature to which it was exposed caused a 

 diminution of the last-mentioned proportion of potash, the actual 

 number obtained, on analysis of No. 3, being 0.512 of potash, and 

 in No. 4 nearly the same quantity of potash namely, .544 grs. 

 were obtained. 



The actual quantities of soda rendered more soluble in burn- 

 ing are trifling, but still sufficiently large to confirm the fact that 

 soda is rendered more soluble in burning. The higher tempera- 

 ture applied in burning No. 3 and No. 4 likewise was attended 

 with a slight diminution of soluble soda, when compared with No. 2. 



No. 1. No. 2. No. 3. No. 4. 



Soda, . . 0.220 0.336 0.314 0.104 



Another important difference in the composition of these 

 four samples of clays which, however, is more interesting in a 

 theoretical than in a practical point of view is presented in the 

 relative quantities of lime which were found in the soluble portion 

 of each. In clay in its natural state, the quantity of carbonate of 

 lime amounted to 0.740 per cent ; in moderately burnt clay (No. 2) 

 to 0.420 ; in clay burnt at a higher temperature (No. 3) to 0.550 ; 

 and in over-burnt clay (No. 4) to 0.188. The three latter quanti- 

 ties are marked down in the analyses as carbonate of lime, for the 

 sake of comparison with No. 1, in which the lime really existed 

 as carbonate of lime ; but as not the slightest effervescence took 

 place on dissolving the burnt clay in dilute muriatic acid, it is clear 

 that the lime did not exist in it in a state of carbonate. The lime 

 must have existed in No. 2, No. 3, and No. 4, as caustic lime, or 

 in a state of silicate ; it would have been, therefore, more correct 

 to indicate the quantity of pure lime in the above table. 



The excess in analyses No. 1, No. 2, and No. 3, is partly due to 

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

 of protoxide of iron, in burning, becomes decomposed. The pro- 

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

 the analyses it is determined and calculated as peroxide of iron ; 

 hence we find the largest excess in No. 2, in which most iron has 

 become soluble in dilute muriatic acid. 



The following considerations induce me to think that the lime 

 in Nos. 1, 2, 3, and 4 existed in a state of silicate. Chemists 

 are well acquainted with the methods of determining the quantity 

 of potash and soda in insoluble silicates, to which class of silicates 

 felspar belongs. 



