17 H Tribasic Phosphoric Acid. 



It will be Been bj these figures thai in the ordinary course 

 of analysis, very satisfactory results may be obtained byreduc- 



• 



ing the iron solution with zinc and Bulphuric acid. Hydro- 

 chloric acid, however, which is ordinarily used in the reduction 

 of iron in iron ores, will lead to errors, on account of its decom- 

 posing some <>t* the permanganate of potassa, and rids will take 

 place even in yery dilute sol uti< >ns, when the odor of chlorine is 

 not perceptible. In precipitating by ammonia, and weighing the 

 precipitate of sesquioxide of iron, traces al leasl of alkalies are 

 always retained, and these it does not seem possible to separate 

 from the hydrated sesquioxide by washing. The same is the 

 case in regard to phosphoric acid, the absolute separation of 

 winch is connected with the greatest difficulties — so much bo, 

 that I feel justified in saying that, so far, no iron has ever been 

 prepared by metallurgical process, or by the most minute and 



careful work oftheac< iplished chemist, which was perfectly 



free from phosphorus. The Bame has been proved with re- 

 gard to sulphur, and is also yery probably true with regard to 

 carbon. We do not know as yel the properties of absolutely 

 pure iron, 



This tenacity of the sesquioxide of iron to retain alkaline sal ts, 

 especially Baits of potassa and ammonia, in connection with the 

 -a i ne <;■ of affinity thai it has for phosphoric acid, seems to 



me a matter of greal moment in the process of nourishment of 

 plants. The hydrated sesquioxide of iron, which, to a greater 

 ..)• less extent, is never wanting in any soil, is, like the humup, 

 a holder of those mineral constituents on the presence of which 

 in the soil is dependent the existence of vegetable life. I 

 may Bay thai when you consider the powers of the see 

 quioxide of iron (as well as alumina) of condensing in its 

 port . such as carbonic acid and ammonia, which gases 



are the daily fond of plants, it is perhaps possible to demon- 

 strate that the fertility of a s<.il depends upon the presence of 

 a certain amount of sesquioxide of iron. Il is certain tli at 

 the latter Bubstance, changing continually a> ii doc-, converts 



