14 



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

 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 all 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. 



Though water, containing carbonic acid, acts more slowly, yet 

 it produces the same effects on the constituents of clay as dilute 

 mineral acids. 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 quanti- 

 ties of Nos. I., II., 111., and IV., were taken for analysis, and each 

 boiled for half an hour in four ounces of water, 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., III., 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 ana- 

 lyses : 



The suggestions to which these analytical results give rise will 

 be more intelligible after we shall have briefly considered the 



