224 



MANURES. 



Sect. X. 7. j 



ed quick vegetation. Journal de Phyfique, 1798. See No. 2. 8. of 

 this Sedion. 



3. When clays are turned up with the fpade, as is ufual in prepar- 

 ino;them for the brick kihi, a kind of efFervefcence occurs, as men- 



tioned above ; which is probably owing to the efcapeof the azote of 

 the air imprifoned in the interlaces, as the oxygen unites with fome 

 metallic particles in the clay; or to fleam ralfed from the water in 

 the clay by the heat fet at liberty from the combination of the oxy- 

 gen and the iron. This union of oxygen with iron is curioufly al- 

 mofl vifible in many granates or porphyries ; which I have feeu 

 thinly fcattered in large nodules near Cannock in Staffordfhire, in the 

 road from Lichfield to Shrewfbury ; and on breaking them have ob- 

 ferved no appearance of iron on the newly divided furfaces ; but which 

 in a few days acquired an ochery appearance on them, which pene- 

 trated nearly half an inch. This can not but be afcribed to the oxv^-en 

 of the atmofphere having united with the iron in thefe ftones, which 

 by their fmell, when breathed upon, contain indurated clay, and hav- 

 ing converted into an oxyde either the clay itfelf, or fome metallic 

 particles included in it. 



There is neverthelefs an exhalation from clay, and perhaps from 

 moft foils, when they have been previoufly dried, and then fprinkled 

 with water, as after a (bower in fummer, which has been fuppofed 



to be falubrious to invalids and convalefcents. 



This remarkably oc- 



curs, when dry clay is breathed upon even in its moft indurated {late, 

 as in granites and porphyries, by which criterion thefe flones are im- 

 mediately diftinguifhed from the filiceous and calcareous ones. This 

 I imagine is produced by the heat fet at liberty by the combination 

 of dry clay and water, like that produced in fo much greater degree 

 by the combination of lime and water ; and that this heat raifes a part 

 of the acquired moifture into fteam, in which are diflblved the odor- 

 ous particles ; both which probably caufe the quick vegetation on 

 clayey foils after the fhowers in fummer. 



When 



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