xvii

acid present. The liquid then assumes a yellow color, and depos-
its a yellow precipitate of a crystalline structure, the formation of
which is accelerated by heating.

This precipitate consists of about three per cent, of phosphoric
acid, a very small quantity of ammonia, and, the rest, of molybdic
acid, which are united to each other in a manner yet doubtful. It
is easy soluble in ammonia; and also in an excess of phosphates ;
insoluble, however, in hydrochloric and nitric acids.

These conditions demonstrate at once, that the application of
this method, is especially adapted to the tracing of small quantities
of Phosphoric Acid as in soils. In this case no excess of phos-
phates, in which the form precipitate could be dissolved, is to be
feared, nor any danger in overlooking a precipitate, which consists
almost entirely of substances used for precipitation, with only a
small portion of the substance which is the object of search.

Direct experiments to test the sensibility of this re-agent were
made, and the following results obtained.

By dissolving 5 grammes of crystallized phosphate of soda,
which contain 1 gramme of Phosphoric Acid, in 25 grammes of wa-
ter, a solution was obtained, which contained in all its parts 1-30
of its weight of Phosphoric Acid.

With this normal liquid, different other solutions were then made,
by diluting the former with water and a suitable quantity of nitric
acid; the respective quantities of Phosphoric Acid in which were
1-300, 1-3000, 1-6000, 1-12000, 1-24000, 1-48000, 1-96000,
1-192000.

After molybdate of ammonia being added, and the whole heated,
1-48000 or 1-50000 of the dilutions created a distinctly vi ibe pre-
cipitate. The 1-96000 dilution formed after a few days a little
precipitate which did not appear at the 1-92000 dilution.

Experiments performed in the same mode, differing only with
regard to the normal liquid, which was in this instance diluted with
a concentrated solution of a soil, in nitric acid, (the soil did not con-
tain any appreciable quantity of Phosphoric Acid) instead of acidu-
lated water, led exactly to the same results, viz: 1-48000 or
1 -50000 dilution caused immediately a distinct precipitate; 1 -96000
after a few days; 1-192000 none at all.

Organic matter, iron clay, lime, and all those substances, which
are extracted by the aid of nitric acid from soils, had exercised not
the least influence on this reaction.

After these remarks, the last but most important point to be con-
sidered is finally the manner in which the above mentioned results
of observations can be applied for the recognition and the quantita-
tive determination of Phosphoric Acid in soils, as distinctly as the
farmer needs knowing it.

The solution of this point leaves now hardly any difficulties.