The draining cost 3, Is., and burning the clay 2, 5s. per 

 acre. The produce was worth 17 per acre. 



Mr Pusey justly observes, that burnt clay does not act merely 

 mechanically, but also as a manure, (that is, chemically.) The 

 reason, however, why it does so, and sometimes fails to do so, he 

 does not attempt to explain. To elicit from the nature of the 

 chemical changes, produced by the process of burning, and from 

 a consideration of the various circumstances under which it may 

 be performed, some solution of those questions has been the object 

 of my investigation. 



Practical men agree that clay-burning is rather a nice operation, 

 and requires much attention and judgment on the part of the 

 operator. It is well known that, if the heat in burning clay is 

 allowed to become too intense, the result will be that, instead of a 

 friable mass, large hard lumps, resembling brick-bats, will be 

 produced, which rather injure than improve the soil. It is fair to 

 infer, therefore, that ignorance or carelessness, in this respect, 

 accounts for the failure of some cases. All failures, however, 

 cannot be attributed to this cause ; for many men, well acquainted 

 with the subject, affirm that some kinds of clay are unfit for 

 burning, because the increase of the crops is not adequate to the 

 expense and trouble of burning and spreading this kind of manure. 

 Now, it is clear that, unless we know the true cause of the effect 

 produced by the application of burnt clay, we are not likely easily 

 to settle, without incurring much expense, which kind of clay is 

 well adapted for burning, and which not. At the same time, the 

 discovery of the cause of the efficacy of burnt clay might lead us to 

 supply the same substances or materials, on which the effects in 

 burnt clay depend, in another form at a cheaper rate. An addi- 

 tional advantage may be derived from this consideration, by which 

 improvements are likely to be effected in the existing methods of 

 burning clay, which shall render them easier, cheaper, and more 

 certain. 



These and similar considerations, I think, will at once show the 

 practical importance of a thoroughly fundamental investigation of 

 this subject. 



The analyses of different samples of agricultural clays, taken 

 from several geological formations, and even those of the same 

 geological epoch, afford great differences in their chemical constitu- 

 ents. Such clays are generally composed of alumina, silica, oxide 

 of iron, manganese, lime, magnesia, potash, soda, traces of sul- 

 phuric and phosphoric acid, and chlorine, in different proportions. 

 It is perhaps just on account of the complexity of composition, 

 and the various changes in -the chemical relation of these different 

 constituents, under so powerful an agent as heat, that the diffi- 

 culty of settling the above questions arises. Room for much 

 speculation is the necessary result of these circumstances ; conse- 

 quently many theories with regard to the effects of burnt clay 



