17 



stances under which the organic matter was not altogether de- 

 stroyed, left 80.260 grs. of insoluble inorganic substances, and 

 furnished 10.580 grs. of soluble inorganic matters. An increase 

 of the temperature, sufficiently high to burn off the small amount 

 of organic matter which enters into the composition of this clay, 

 had the effect of producing the solubility of its constituents to 

 about li per cent ; and a more protracted exposure to a still 

 higher temperature had the effect of a further reduction of 

 its solubility, to such an extent that this over-burnt clay became 

 less soluble than the same clay in its natural state. 



PROPORTION OF 



Soluble inorganic Insoluble mineral 

 matter. matter. 



Clay, No. 1, (unburnt,) .... 6.740 84.100 



Clay, No. 2, (slightly burnt,) . . 10.580 80.260 



Clay, No. 3, (burnt stronger than 2,) . 8.955 81.845 



Clay, No. 4, (over-burnt,) . . . 5,391 85.309 



It is difficult to determine at what exact temperature clay 

 should be burnt for agricultural purposes, and I am inclined to 

 believe that this point cannot be settled by small experiments in 

 the laboratory. Besides, if it could be done, the mere indication 

 of the degree of heat would not be a sufficient guide to the practi- 

 cal man, and therefore possesses little more than a theoretical 

 value. Valuable results, however, might be obtained by recording 

 the exact circumstances under which different clays have been 

 burnt, by observing the practical effects of clay burnt in different 

 ways, and reserving a portion of each sample of clay for chemical 

 analysis. The analysis of a series of different clays, aud clays 

 burnt in different manners, I have no doubt, besides .throwing 

 additional light on the rationale of the process of soil burning, 

 is likely to be attended with important practical results. 



3. What is the relative composition of the soluble matter in 

 clay burnt in different manners V 



Referring to the above analytical results, we find much differ- 

 ence in the composition of the soluble portion of each of the four 

 samples of clay ; but I would invite particular attention to the 

 important fact, which is distinctly proved by these analyses, that 

 the proportion of alkalies, more particularly that of potash, is much 

 larger in the burnt than in the unburnt clay. Finding the 

 quantity of so valuable a fertilising substance as potash very much 

 increased in the soluble portion of burnt clay, and considering 

 that this is precisely the effect produced in fallowing, as demon- 

 strated above, I have no hesitation to assign the chief cause of the 

 beneficial effects of burnt clay to a larger quantity of potash 

 which is liberated by burning and rendered available for immediate 

 use by plants. 



The temperature to which the clay has been exposed, here 

 regulates the proportion of potash rendered soluble in dilute 



