182 Prof. Davy on a Simple and Expeditious Method of 
position, viz. (Fe? 0%, PO*), in which 80 parts of the peroxide 
containing 56 of metallic iron, are combined with 72 parts of 
phosphoric acid. 
The fact of the peroxide of iron forming an insoluble com- 
pound with phosphoric acid has long been known ; and different 
methods founded on it have been proposed and adopted for the 
estimation of phosphoric acid and its compounds,—as, for ex- 
ample, those of Berthier, Kobell, and Raewsky, which are described 
in different works on analytical chemistry. 
Each of those methods, however, requires a considerable 
devotion of time, from the collecting, washing, drying, igniting, 
weighing, and other operations through which the precipitated 
phosphate of iron has to pass, which not only consume much 
time, but, unless they are very carefully performed, lead to great 
inaccuracies in the results obtained. 
In the modification I propose, I dispense altogether with those 
tedious and troublesome operations, by simply adding a gra- 
duated solution of iron of known strength to the phosphate, and 
ascertaining the point when sufficient iron has been added to 
combine with all the phosphoric acid present; and from the 
quantity of iron employed, I calculate the amount of that acid ; 
every 56 parts of iron being equivalent to 72 of phosphoric 
acid. 
The iron solution which I use for this purpose is somewhat 
different from that hitherto employed. I make it in the following 
manner: a certain quantity of the finest pianoforte iron wire, 
perfectly clean and free from rust, is dissolved in pure hydro- 
chloric acid, and sufficient nitric acid is afterwards added to 
convert the so-formed protochloride into the perchloride of iron ; 
and as any excess of hydrochloric acid would be injurious to the 
process, and as it cannot be removed by heating the mixture and 
evaporating it to dryness, which would decompose a portion of 
the perchloride, giving rise to peroxide of iron and hydrochloric 
acid, I add caustic ammonia till all the free acid has combined 
with that substance, and a small quantity of the peroxide of iron 
precipitated by the alkali remains undissolved after agitating the 
mixture and allowing it to stand for a few minutes. Acetic acid 
is then added to dissolve the oxide, and when it has effected its 
complete solution (which it will do by leaving the acid to act on 
the oxide at the ordinary temperature for a short time), the mix- 
ture is largely diluted with distilled water and graduated in the 
usual way, so that the amount of iron may be known which is 
contained in a given quantity of the liquid. The proportions 
which I have used are 100 grains of iron in 1000 cubic centi- 
metres of the liquid ;: and this quantity of standard solution will 
suffice for a great number of determinations. 
