Oxidation of Iron in Soils. 441 



position. The use of the red oxide of iron in a soil has been 

 stated to be its power of retaining ammonia. 1 must confess, 

 however, I am rather inclined to doubt that it is of any great 

 value to it, on account of this property, as all soils containing 

 much of it are found to be of poor quality; but, admitting 

 this to be the fact, there does not appear to me to be any rea- 

 son to doubt that this retentive power is equally possessed by 

 the protoxide. 



From the above observations I was led to the conclusion, 

 that the preservative action of the humus on the protoxide of 

 iron in soils was probably analogous to that of sugar in some 

 pharmaceutical preparations, particularly in that of the sac- 

 charated carbonate of iron of the Edinburgh Pharmacopoeia. 

 I therefore prepared some of it by the process there given, 

 and found, on passing a current of air over a portion of it by 

 the apparatus I have before described, that a small amount of 

 carbonic acid was given out from it. I do not, however, wish 

 to be considered as speaking positively as to this being the 

 action of the sugar, although the experiment shown above 

 would appear to render it probable. 



Note. 



Since writing the above communication, I have had brought 

 before my attention a remark occurring in several agricultu- 

 ral works, that some clays require either burning or long ex- 

 posure to the atmosphere before they are fitted to be mixed 

 with fertile soils ; and as the iron in them is found to become 

 peroxide during these operations, it has been brought forward 

 as another proof of the injurious action of the protoxide of iron. 



In my opinion, however, it is not because the iron is in the 

 state of protoxide that clays require this treatment, but on ac- 

 count of its existing as sulphuret, which, as is well known, is 

 hurtful to vegetation ; and I believe that the appearance of 

 oxidation assumed by them during exposure to the atmosphere, 

 is nothing more than the decomposition of the sulphuret into 

 peroxide of iron. That this decomposition would take place 

 under these circumstances I may instance, in the case of an 

 embankment of clay on the Croydon railway, where, accom- 

 panied by the formation of sulphate of lime, it occurred so ex- 

 tensively as to destroy part of the work. 



I have, on analysis, usually found deep clays to contain sul- 

 phuret of iron ; and another proof of its existence in them may 

 be adduced from the slaty clay usually accompanying the coal 

 formation, where it is found so extensively as to be employed 

 for the manufactures of alum and copperas. 



Craig's Court, April 9, 1845. 



