2 Q UANTITA TIVE AGRICULTURAL A NAL YfUfi

ammonium phosphate, MgNH4P04, which is washed and then
changed to magnesium pyrophosphate, Mg2P2O7, by heating
strongly in a previously weighed crucible. From the weight of
the crucible, with and without the pyrophosphate, the weight of
the latter is found.

Factors.— The formula for magnesium pyrophosphate shows

(2P X 100 \

____ = 27.87 j.

Multiplying this figure by the weight of pyrophosphate found
and dividing the product by the weight of sample gives the pot-
cent of phosphorus in the phosphate rock. Stated a>s a formula :

2 X 3X° W

where W = grams of magnesium pyrophosphate found and
8 = grams of sample taken. No matter how many different
samples' of rock or other material might be subjected to this
experimental process, the calculation would always follow the
lines indicated in Eq. (1) and, since the only variables in this
equation are the weights of sample and of pyrophosphate, the
constants may be collected :

2 X 31.04 X 100 _ _ -,

The quantity F is called a "gravimetric factor" and, since the
procedure for phosphorus as already outlined is an illustration of
the procedure for all gravimetric determinations, this factor may
be calculated once for all for each type of determination arid
recorded, together with its logarithm, in a convenient place*.
Equation (1) is then a special application of the more general
equation:
F W ' /<>A
-g~ = ^ Oi)
-F always indicating the per cent of the determined element or
combination of elements in the weighed precipitate, an calcu-
lated from the chemical formula, and x representing the per cent
of the same entity in the sample analyzed.
As indicated in the preceding paragraph, a combination
of elements (as an oxide or radical) may be calculated. For
example the factor for phosphorus pentoxide would be
100 F205 _ 14208 _
Mg2P207 " 222.72 " W'7y'