498 On the Causes of the Efficacy of Burnt Clay. 



In engaging in this inquiry I proposed to myself the following 

 questions : — 



1. What are the changes produced in clay upon burning? 



2. How does burnt clay act in improving the soils, or, in other 

 words, what are the causes to which the efficacy of burnt clay is 

 due ? 



3. By what qualities or characteristics are clays totally unfit 

 for burning, distinguished from those which prove most efficacious 

 after burning ? 



4. Can it be determined by chemical analysis whether burn- 

 ing will be efficacious in rendering clay a fertilizer ; or not ? 



5. What are the reasons of the failures attending over- 

 burning ? 



6. What is the reason that burnt clays improve especially 

 root- crops ? 



Each of these questions I shall endeavour to answer separately: 



1 . What are the changes produced in Clay upon Burning ? 



When heavy clay land is burned at a proper temperature it is 

 materially altered in its physical condition, whilst at the same 

 time the action of the heat produces some chemical changes in 

 the constituents of clay, which appear to me highly important. 

 I shall consider : — 



I. The Mechanical Effects of Heat on Clay. — The success of 

 the operation of clay-burning depends much on the proper regu- 

 lation of the temperature, and the operation itself requires, 

 therefore, much skill and constant attention on the part of the 

 operator ; for if burnt at too high a temperature, or overburnt, 

 clay melts or is converted into large stony masses, which do not 

 crumble to powder under the action of the weather. Thereby 

 not only the benefits of the heat in altering the mechanical con- 

 dition of ihe soil are lost, but also, as I shall show presently, the 

 benefits arising from a change in the chemical nature of burnt 

 clay. 



The mechanical effects of heat on clay are simple and easy to 

 be understood. Heavy, stiff clay soils are impervious to water, very 

 tenacious, and unctuous, and, for these reasons, often cold and 

 expensive to work. Clay, when properly burnt, so that it 

 neither melts nor becomes converted into hard, stony masses, is 

 rendered harder to some degree, more porous, and, under the 

 action of the weather, crumbles down to powder, without again 

 becoming tenacious or plastic. 



The application of burnt clay to heavy soils thus tends to 

 make them more open and more friable, and consequently dimi- 

 nishes much the amount of labour in the subsequent working of 



