16 



manure. The high price of salts of potash unfortunately prevents 

 their more extensive application in agriculture, and plants are, 

 therefore, dependent in a great measure on the natural sources 

 from which they derive their potash. The chief source of potash 

 in ordinary soils is the clay, which forms part of almost all soils, 

 and which itself usually contains some undecomposed silicate of 

 potash or a duple silicate of potash or soda and an earthy base, 

 from which, in gradual decomposition, potash is set free and made 

 available to plants. Clay, we have seen, is in many cases derived 

 from felspar : the more undecomposed felspar-fragments a clay 

 contains, the more it is likely to prove useful to plants. Hence we 

 are enabled to explain the advantages of fallowing. By that 

 process a fresh portion of the soil, not hitherto exposed to the 

 action of the atmosphere, is brought up, and the undecomposed 

 fragments of felspar are forced by the combined action of air and 

 water, to yield their potash and soda, which are the indispensable 

 requisites of a healthy vegetation. 



Without doubt, then, potash is the most valuable substance in 

 clays, and, if I am not mistaken, the substance on which their 

 manuring qualities mainly depends. 



In an age of railway and steamboat enterprise and telegraphic 

 despatch, agriculture is forced to progress, and, in consequence of 

 this, fallowing must necessarily yield to some more extensive and 

 expeditious means of gaining the same advantages. Now, I am 

 prepared to show, that, in burning clay, precisely the same changes 

 are effected in a few days which in bare-fallowing are produced 

 in so many months : in other words, the natural fertility of the 

 soil, which in fallowing is restored after a long interval of rest, 

 can be restored hi many instances in a few days, by burning 

 land. 



Let us,, however, examine the proposed questions separately. 



1. Is the clay from Huntstile, near Bridgewater, more soluble 

 in dilute hydrochloric acid, after than before burning? 



A reference to the above tabulated analytical results will show 

 that, after burning, this clay has become much more soluble than 

 the clay in its natural state. 



2. What are the relative proportions of soluble and insoluble 

 matters in this clay when burnt in different manners ? 



The above-mentioned results not only teach, generally, that 

 clay becomes more soluble in burning, but that the temperature 

 to which it is exposed mainly regulates the solubility of the clay. 

 A proper temperature for burning clay is, indeed, a condition in 

 the process, on which the success of the operation chiefly depends. 



We see, from the preceding tabulated results, that clay, in 100 

 parts in its natural state, furnishes only 6.74 grs. of soluble 

 inorganic matter, leaving 84.100 insoluble mineral matters behind ; 

 whilst the same clay, burnt at a temperature, and under circum^ 



