h, combining with the carbonic acid, forms carbonate
of ammonia, a very volatile salt. It is to this fact that one of the
great sources of loss in the decomposition of farmyard manure is due. If
the temperature of the manure-heap be permitted to rise too high, the
carbonate of ammonia volatilises. It is probable, also, that a not
inconsiderable portion of the nitrogen escapes into the air in the free
state. The last of the most important gaseous products of decomposition
are _sulphuretted_ and _phosphoretted hydrogen_. It is to these gases
that much of the smell of rotting farmyard manure is due.

2. The second class of substances formed are _soluble organic acids_,
such as _humic_ and _ulmic acids_. The function performed by these acids
is a very important one. They unite with the ammonia and the alkali
substances in the mineral portion of the manure, forming humates and
ulmates of ammonia, potash, &c. It is these ulmates that form the black
liquor which oozes out from the manure-heap.

In very rotten farmyard manure traces of _nitric acid_ may be found; but
it must be remembered that the formation of nitrates is practically
impossible under the ordinary conditions of active fermentation of
farmyard manure, except perhaps in its very last stages.

3. The third class of changes taking place have to do with the mineral
portion of the manure. The result of the formation of so much carbonic
and other organic acids is to increase the amount of _soluble_ mineral
matter very considerably.

_Analyses of Farmyard Manure._

It is chiefly to the valuable researches of the late Dr Augustus
Voelcker that we owe our knowledge of the composition of old and fresh
farmyard manure. All interested in this important question should peruse
the original papers on this subject contributed to the 'Journal of the
Royal Agricultural Society' by Dr Voelcker. Typical analyses
illustrating the variation in the composition of farmyard manure at
different stages of decomposition will be found in the Appendix.[164]
From what has been already said, it is obvious chat the composition of
farmyard manure is of a very variable nature.

The quantity of moisture naturally varies most, and this variation will
depend on the age of the manure, and the conditions under which it is
permitted to decay. It may be taken at from a minimum of 65 per cent in
fresh to 80 per cent in well-rotted manure. The total organic matter may
be taken at from 13 to 14 per