On the Causes of the EfficacTj of Burnt Clay. 511 



1. The mechanical chansfes produced on clay upon l^urning, 

 which bv no means are unim'portant, nevertheless do not suiii- 

 ciently explain the fertilizing effects of burnt clay. 



2. These are dependent on the chemical as well as on the 

 mechanical changes both produced upon burning clay. 



3. The fertilizing effects of burnt clay are mainly dependent 

 on the larger amount of potash which is liberated from the inso- 

 luble silicates of the clay in the process of burning. 



4. Burnt clay contains more soluble potash than the same clay 

 unburnt. 



5. Clay after burning becomes more soluble in dilute acids. 



6. The temperature used in burning clay regulates the solu- 

 bility of clay; too intense a heat renders clay again less soluble. 



7. A temperature whereby the organic matter in clay soils is 

 merely charred, but not altogether destroyed, should be employed 

 in burning clay in the field. 



8. Properly burnt clay furnishes a larger proportion of soluble 

 potash and soda than clay burnt at too high a temperature. 



9. In burning clay similar effects are produced as in bare- 

 fallow. 



10. Clays, originally containing a large proportion of unde- 

 composed silicates of potash and soda, are best suited for burning. 



11. On the contrary, those resembling in composition pure 

 pipe or porcelain clays, and all those which contain mere traces 

 of undecomposed alkaline silicates, are unfit for burning. 



12. It is desirable that clay which is intended to be burnt 

 should contain lime. 



13. The application of quick-lime to newly-burnt clay land, or 

 the mixing of clay with lime before burning, is likely to be 

 attended with much benefit. 



14. Overburnt clay does not absorb so much ammonia from 

 the atmosphere as properly burnt clay. 



15. The causes of the failures attending overburning are 

 due : — 



1. To the mechanical changes which clay undergoes in 

 overburning, whereby it is rendered hard, like stone. 



2. To the chemical changes, whereby the constituents of 

 clay are rendered less soluble. 



3. To the diminished porosity, and consequently reduced 

 absorptive power, of such clays. 



16. Burnt clay improves especially turnips, carrots, potatoes, 

 and other green crops, because it supplies potash, which these 

 crops largely require, more abundantly and more readily than 

 unburnt clay. 



