estimating Phosphoric Acid and its Compounds. 185 
Be this as it may, my experiments have clearly shown me that 
under the conditions in which I place those substances, in the 
method recommended for the estimation of phosphoric acid, a 
compound having the composition (Fe? 0%, PO®*) is uniformly 
produced. I have proved this by taking certain quantities of 
different phosphates and treating them in the manner described, 
I ascertained how much iron was necessary to combine with the 
phosphoric acid present ; and in every case I have obtained re- 
sults which agree almost exactly with those I should have got, 
calculating according to that formula, which would not have 
been the case had the composition of the phosphate of iron been 
different. 
The following are taken from among my experiments. Some 
pyrophosphate of magnesia and tribasic phosphate of lime being 
carefully prepared, a certain quantity of each was taken and dis- 
solved by the aid of heat in a little hydrochloric acid; and the 
solutions being diluted with distilled water, they were very care- 
fully graduated, so that 5 cubic centimetres should contain one 
grain of each of those compounds. 
Common tribasic phosphate of soda was likewise taken, and 
after being exposed to a red heat for some time to convert it into 
the anhydrous pyrophosphate, a solution of it was also made, con- 
taining the same proportion of dry salt as in the former cases. 
Five cubic centimetres of each solution (containing one grain 
of those compounds) were then taken and several estimations 
made, employing the same quantity of solution every time, when 
the results obtained were as follows :— 
Amount of iron required to combine with the phosphoric acid 
contained in one grain of— 
By Calculation, By Experiment, 
parts of a grain. parts of a grain. 
0°5000 1st Experiment 0:5000 
Pyrophosphate of magnesia ” 2nd yt 0:5000 
” 3rd 9 0°5000 
” 4th %3 0-5000 
03589 Ist |;  0-3600 
og ' aitaty aaa eek 0:3600 
Tribasic phosphate of lime : Bnd ss 03600 
spaipsedtliniucrsy tis 00-3600 
04179 Ist , 0:4200 
Anhydrous pyrophosphate 3 2nd 55 0:4200 
of soda* ‘% 3rd yi 0°4200 
pie Om? ON. 00? Oi nRe ey 
* Inthe case of the bibasic salts, it is necessary to convert them into the 
tribasic before the addition of the iron solution; this I have found (in the 
