oned that Boussingault has found by experiment
that the total percentage of nitrogen contained by fresh horse-manure
might be reduced in the process of fermentation to one-half of its
original amount by loss from this source.

The preservatives used to prevent this volatilisation are technically
known as "fixers." This they do by chemically combining with the
volatile ammonia and forming non-volatile compounds with it.

Of the acid fixers, hydrochloric and sulphuric acids have been
recommended. The former, however, is not well suited for this purpose.
It is a strongly fuming acid, and when brought into contact with
ammonia forms dense white fumes. The use of sulphuric acid is not open
to this objection. Sulphate of ammonia, the salt formed in this case, is
one of the most stable (or least volatile) of the compounds of ammonia.
If used, it should be largely diluted with water, and the whole mixed
with sand. Such a mixture, when sprinkled over the stable-floor in even
very small quantities, has been found to effectually prevent any loss of
the volatile carbonate of ammonia.

It is not, however, on the whole advisable to use an acid substance as a
fixer, since such a substance may act deleteriously on the horses'
hoofs.

Such substances as _gypsum_, _copperas_, and _sulphate of magnesia_,
while equally efficient, are not open to this objection. The
above-mentioned substances owe their efficacy to the fact that they are
compounds of sulphuric acid, which, by combining with the volatile
ammonia and forming sulphate of ammonia, prevent its escape.

Gypsum, or sulphate of lime, although, comparatively speaking, an
insoluble substance, when brought in contact with carbonate of ammonia
has been proved to effect the conversion of the ammonia into sulphate of
ammonia. It is also believed to retard the decomposition of the
manure.[155] Copperas, or ferrous sulphate, while a soluble salt, and
while thus acting in a more speedy manner in fixing the ammonia, is not
so well suited, owing to the hurtful influence it is well known to
possess on plant-life. It is only right to remember that there may be
circumstances in which copperas may, in small quantities, act even
beneficially as a manure, as Griffiths' experiments would seem to
indicate. The above objection, however, cannot be urged against sulphate
of magnesia. In addition to fixing the ammonia, sulphate of magnesia may
very probably fix the soluble phosphoric acid. Kainit, wh