* 212 DR. DAUJBENY ON THE ROTATION OF CROPS, ETC. 



to be burnt being too considerable to allow of its calcination in any of the platina 

 vessels which I chanced to possess. 



The ashes, therefore, of which 200 grains were usually taken, had first to be treated 

 with pure muriatic acid, and the latter to be driven off by heat, so that the silica of 

 the ash might be rendered insoluble. 



Water aitd muiiatic acid were tben added to tli« dry mass, and the portion which 

 <lid not dissolve was separated by filtration. Its weight, after being washed, repre- 

 sented the amount of silica in the ash, together with that of the extraneous sand and 

 charcoal intermixed with it. The former was separated by digestion in pure dilute 

 alkaline ley, and its quantity determined, in the first place indirectly, by the loss of 

 weight sustained by the insoluble portion after its removal, and in the second more 

 accurately, by the direct process of separating it from its solution in the alkali by 

 treatment with an acid, and subsequent evaporation to dryness, after which, water 

 having been added in sufficient quantity to redissolve the alkaline salt, the silica was 

 collected on a filter, and then dried and ignited previously to weighing it. 



The solution in dilute muriatic acid was made up to some definite quantity, so that 

 it might be divided into four exactly equal portions, of which one was kept in reserve 

 in case of any accident happening to the remainder, whilst the three others, which 

 we will call A, B and C^ were examined for the different ingredients present, as for 

 instance, — • 



A. For the peroxide of iron. 



B. For the phosphoric acid. 



C. For the alkalies. 



In most of the parts of vegetables, especially in their seeds, and in the tubers and 

 bulbs which afford nutriment to animals, the amount of phosphoric acid may be ex- 

 pected to exceed that necessary for combining with the iron present. A reagent then 

 which throws down phosphate of iron, affords us in these cases a ready means of 

 estimating the whole amount of that metal, from the weight of phosphate obtained, 

 and Will assures us* that 100 grains of the latter precipitate consists of 43-92 phos- 

 phoric acid, and 56-08 peroxide of iron. 



When therefore, to the muriatic solution A, containing a slight excess of acid, ace- flHI 

 tate of ammonia is added, the muriatic acid, which had held the phosphate of iron in ^^1 

 solution, is seized upon by the ammonia of the former salt, and the phosphate of iron, ^' 



being insoluble in the liberated acetic acid, is precipitated. 



We thus obtain a means of readily estimating the amount of peroxide of iron, but 

 not of determining that of phosphoric acid, because there maybe still a portion of the 

 latter remaining in the liquid in combination with other bases, the phosphate of lime 

 and of magnesia being soluble in free acetic acid, and the alkaline phosphates being 

 so even in water. 



In order therefore to estunate the amount of phosphoric acid, an expedient was 



* Memoirs of the Chemical Society, part 9. 



