11 



dental constituents of agricultural clays as the chief causes of the 

 fertilising effects of burnt clay. Without giving any experimental 

 support to his theory, he reasons, with much probability, by analogy, 

 that some of these accessory constituents of clay, particularly 

 potash, soda, lime, and magnesia, are rendered more soluble in 

 the process of burning. To this circumstance he ascribes the chief 

 causes of the effects of burnt clay. It is curious that his theory, 

 set forth with much perspicuity and ingenuity, was by no means 

 generally well received at the time of its publication. Nevertheless, 

 Zierl's theory seems to me the most rational of all the theories 

 which have been advanced. Without a previous knowledge of 

 Zierl's paper, or the causes of the effects of burnt clay, I have 

 formed a theory which, in many respects, agrees with his ; and 

 when I shall bring forward the facts by which I hope to support 

 my theory, 1 shall point out the importance of Professor Zierl's 

 speculations, which, unfortunately for him, were not borne out by 

 any testimony or experiment. 



It now remains for me only to say a few words about the opinion 

 which Professor Hermbstadt advanced in a paper, which appeared 

 in Erdmann's Journal for 1 833, vol. i. p. 45, concerning the effects 

 of burnt clay. His views on the use of inorganic constituents of 

 the soil to plants are so entirely at variance with the generally 

 accepted opinions of chemists and physiologists, that the endeavour 

 to refute them might appear as a waste of words and of time. 

 The same remark applies to his views concerning the effects of 

 burnt clay ; and we shall, therefore, only mention, that Professor 

 Hermbstadt refers the active principle of burnt clay entirely to the 

 organic matters which have not been destroyed by the fire. For 

 obvious reasons, he is no advocate of the use of burnt clay ; and, 

 although Lainpadius's, and many other practical experiments, were 

 published in 1833, he prophesies a total failure to General Beatson's 

 recommendations. Inconsistent as his own views were with the 

 state of science in 1833, he charges General Beatson with ignorance 

 of the first principles of the theory of manures, and this in terms 

 which cannot be too highly deprecated. Ought not his example to 

 make us more charitably inclined towards the opinions of others 

 more guarded and milder in our expressions, and less confident and 

 dogmatic in propounding our own views \ 



From these remarks the reader will perceive that none of the 

 above-mentioned theories explains satisfactorily the cause of 

 the decidedly beneficial effects of burnt clay ; that Sprengel's, 

 Lampadius's, and Kersten's theory, concerning the use of ammonia, 

 and the modified ammonia-theory of Liebig, are open to serious 

 objections. Furthermore, none explains in the least why certain 

 clays, when properly burnt, act more beneficially than others, and 

 what the reasons are which explain the failures attending the 

 application of over-burnt clay. 



Most living agriculturists and agricultural chemists have adopted 

 Liebig^s views respecting the nature of the action of burnt clay, 



