Obstructions in Draining-Tiles. G29 



persal of the carbonic acid, as in the first case, will prevent the 

 entrance of the oxygen of the air. If a little of the gas reaches 

 the tiles during great droughts, or with the water of the first 

 rains, some deposits, it is true, may accidentally be formed, but 

 they will re-act upon themselves after having absorbed the oxygen 

 contained in the air of the tiles, they will speedily return to a 

 soluble state, and they will be easily carried along by the move- 

 ment of the water in the drains during the rainy season. 



It is superfluous to add that drains formed in land subject to 

 produce these ferruginous obstructions ought to be executed with 

 more than ordinary care. The refilling of the trenches ought 

 above all things to be attended to. The most argillaceous parts 

 of the soil should be chosen to place over the tiles ; these should 

 be chopped fine and carefully rammed in the most perfect man- 

 ner. 'I'he composition which forms the basis of these ferruginous 

 incrustations in drain-tiles is found in great quantities in soils 

 circumstanced as above described. It is equally found, but in 

 smaller proportions, in many other soils. It probably plays an 

 important part in the phenomena of vegetation. It is not im- 

 possible, indeed, that it is in this particular state of combination 

 that iron introduces itself into the tissue of plants. It is very 

 probable that ammonia may be formed during the oxydation of 

 this substance, as it is formed when iron rusts in the damp air. 

 The experiments which I have in hand may, I hope, place 

 beyond doubt tliis reaction so interesting for agriculture. 



The chemists who have spoken of the ferruginous obstructions 

 of drains, supposed with reason that these deposits were due to 

 the oxydation of the salts of the protoxide of iron. It was 

 generally thought, however, that they were formed by the preci- 

 pitation of a certain quantity of the carbonate of the protoxide of 

 iron, produced in the bosom of the earth by the action of" the 

 organic materials on the peroxide of iron, and held in soUition in 

 the water by an excess of carbonic acid. Tlie solubility of the 

 carbonate of the peroxide of iron is insufficient to explain the 

 abundance of some of these deposits. No one, moreover, had 

 demonstrated positively the absorption of the oxygen, and no one 

 had oI)served the spontaneous r((huti()n of the deposit, which 

 completely insures the success ol the air-traps of which I have 

 now jiolnted out the use to prevent the ochreous obstructions in 

 drain-tiJQs. 



HERvfc Mangon, 



The draining operations in which I have liecn engaged have 

 been chiefly confined to (lay soils, and 1 have no experience of 

 the difiiculties caused either by the calcareous or the ferruginous 



VOL. XVII. _ 2 u 



