On the Causes of the Efficacy of Burnt Clay. 



501 



By my own experiments, I am enabled to confirm Johnston's 

 observations respectino^ the greater solubility of burnt clay, and 

 the diminution of this solubility in over-burnt clay. I have 

 found further the important fact that, in burning clay properly, a 

 large amount of potash is liberated from the constituents of the 

 clay and rendered soluble which existed before burning; in the 

 clay in an insoluble state. I am indebted for the material with 

 which my experiments were made to Sir Thomas Tancred, who 

 procured for me some clay of the new red sandstone formation 

 from the farm of Huntstile, near Bridgeuater^ tenanted by Mr. 

 Thomas Danger. 



The nature of the chemical changes which may be supposed 

 to affect the action of burnt clay on the land to which it is 

 applied, was examined by four distinct analyses :- — 



No. I. Clay-soil in its natural state. 



No. II. A quantity of the same clay-soil was exposed to a dull 

 red heat in a closed platinum crucible, and kept at that tempera- 

 ture for half an hour. The clay, after burning, had a dark-grey 

 colour. 



No. III. Another portion of the same clay-soil was exposed 

 to a red heat for half an hour in an open crucible. The contents 

 of the crucible were frequently stirred with a platinum wire, in 

 order to effect the complete combustion of the organic matters, 

 and to secure the perfect oxidation of any protoxide of iron 

 which was present in the clay. After burning, the colour of this 

 portion of the clay was red, rather brighter than the natural 

 colour of the soil. 



No. IV. A fourth portion of the same clay-soil was exposed 

 for about three hours to a full red heat in an open crucible. 



As water, saturated with carbonic acid, produces the same effects 

 on the constituents of clay as dilute acid, and acts only slower, I 

 preferred to apply dilute muriatic acid, instead of water charged 

 with carbonic acid, in order to test the solubility of the above four 

 samples of clay. Accordingly, separate quantities of Nos. I., IL, 

 III., and IV. were boiled for half an hour in four ounces of 

 dilute acid, containing one-tenth of its bulk of hydrochloric acid. 

 The insoluble part of the clay was collected on a filter, and 

 washed with distilled water until nothing more was dissolved. 



In the soluble part of Nos. I., II., 111., and IV., the following 

 substances were determined quantitatively : soluble silica, oxide 

 of iron and alumina, carbonate of lime, potash, soda, and phos- 

 phoric acid. 



In No. IV. phosphoric acid was not determined. 

 The following Table exhibits the results of these several 

 analyses : — 



