8 



on vegetation, whilst protoxide of iron is rather injurious to many vege- 

 tables. 



(4.) In burnt clay, ammonia is formed when exposed to the atmosphere in a 

 moist state. 



In his last concluding paper on this subject, Lampadius advanced 

 finally the following theories : 



a. Plants are supplied by burnt clay with humates of alumina and silica. 



6. In burnt clay, exposed to moist atmospheric air, ammonia, which is beneficial 

 to vegetation, is formed according to his own, Sprengel's, and Kersten's obser- 

 vations. 



c. According to Zierl, accessory constituents of clay, as phosphoric acid and pot- 

 ash, contribute to the fertilising effects of burnt clay. 



These, then, are the theoretical opinions advanced by Lampa- 

 dius, and we shall see presently how far they are consistent with 

 the present state of science ; but, in order to avoid repetition, we 

 shall first briefly state the opinions of others who have written on 

 the subject, and shall then submit the various theories, which all 

 more or less agree with Lampadius, to a short review. 



Karl Kersten, who analysed a sample of clay, both in its natu- 

 ral state and burnt, could detect no material difference in the 

 composition of the burnt and the natural clay. The only differ- 

 ence which he points out consists in a slightly greater amount of 

 soluble matter the amount of soluble substances in the natural 

 clay being 0.20 ; in the burnt, 0.30. On so small a difference no 

 theoretical speculation can be built, inasmuch as the unavoidable 

 errors in good analyses fall between these limits. Kersten likewise 

 observed the formation of ammonia in burnt clay which had been 

 exposed to the atmosphere for some time, and inclines to ascribe 

 to the latter the chief fertilising effects of burnt clay. No men- 

 tion is made of the presence of ammonia in the natural clay. 



Professor Kastner thinks clay, when burnt, absorbs light, which 

 being given off again in the soil, exercises a beneficial effect on 

 vegetation ; but as his theory is founded on no experimental 

 proof, we can dismiss it without any further inquiry into the pro- 

 bability of the explanation he has given. 



Dr SprengePs extensive researches on a multitude of chemico- 

 agricultural subjects amongst others, on the causes of the benefi- 

 cial effects of burnt clay are valuable contributions to scientific 

 agriculture. The theory which he first advanced enjoyed the 

 approbation of many of his contemporaries, and is partly still 

 entertained by no less an authority than Liebig, and other 

 eminent chemists of the present day. This theory is generally 

 called the ammonia-theory. According to it, the chief fertilising 

 agent in burnt clay is ammonia, which Sprengel supposes to be 

 formed in burnt clay under the influence of protoxide of iron, from 

 the elements of water and atmospheric air, and which, according 

 to Liebig and others, exists ready formed in the atmosphere, 

 whence it is simply absorbed by the clay. The clay being more 



