DR. DAUBENr ON THE ROTATION OP CROPS, ETC. 213 



adopted, by which the presence of sufficient iron to carry down the whole of the 

 phosphoric acid might be secured. 



For this purpose a certain known weight of clean iron wire wa» dissolved in a 

 mixture of nitric and muriatic acids, care being taken that no loss should occur from 

 the violence of the action occasioned. 



The solution thus prepared will then contain a definite amount of peroxide of ipon, 

 which, when introduced into the liquid containing the ash, wilJ seize upon all the 

 phosphoric acid, not already combined with iron, which it may contain. 



Accordingly, after adding it to the latter, from which the phosphate of iron ongi- 

 nally present had been previously thrown down by acetate of ammonia, we recover 

 the whole of the metal, whether in combination with phosphoric acid or not, by ap- 

 plying again this same reagent, provided only the solution be rendered neutral by 

 ammonia, and raised to a boiling temperature. 



In the case supposed, therefore, the precipitate will indicate the whole of the phos- 

 phoric acid existing in the fluid, after the phosphate of iron originally present had 

 been thrown down, together with the peroxide of iron which results from the iron 

 introduced into it in union with chlorine. 



Accordingly the quantity of phosphoric acid remaining after the first operation 

 may be estimated, by deducting the weight of percxide of iron, which is known, from 

 that of the entire precipitate collected. 



In practice however it was found most convenient to determine the amount of 

 phosphoric acid, by taking another measured portion of the solution, viz. B, and 

 adding to it in the first instance the known weight of iron. We are thus enabled, by 

 following the steps above pointed out, to throw down, all the iron originally present 

 in combination with phosphoric acid, all the phosphoric acid which may have existed 

 in combination with other bases, and the whole of the peroxide of iron, whether pro- 

 ceeding from the ash, or introduced from without. 



The amount of the former portion of the iron will have been ascertained by the ex- 

 amination of the solution A, whilst that of the latter can be readily calculated, as we 

 know the weight of the iron introduced ; by deducting therefore the sum of these two, 

 which represents the total amount of peroxide of iron, from the entire weight of tl>e 

 precipitate, we obtain that of the phosphoric acid present in the ash. 



This modification of the process saves some trouble, as it obviates the necessity of 

 reducing the bulk of the solution remaining after the separation of the phosphate of 

 iron precipitated from A. in the first process, which, owing to the number of washings 

 necessary, becomes inconveniently large. 



I felt curious to ascertain whether the phosphoric acid obtained by the above me- 

 thod was combined with two or with three atoms of base, as Will and Fresenius 

 state, that the Cerealia generally present it in the former predicament, the Legumi- 

 nosee in the latter. 



Our experiments on this point do not appear to confirm such a conclusion, showing 



2 f2 



