Chemical Ckawjcs in the Fermentation of Duny. 325 
process is proportional to the amount of these nitrogenized sub- 
stances and tlie freedom with which they are allowed to act. 
An ordinary manure heap is formed of materials which have 
already advanced a considerable way towards decomposition, and 
are every moment proceeding in the same direction. 
The food of men and animals contains a considerable quantity 
of nitrogen in the form of albuminous compounds, beside carbon 
in the form of starch, sugar, &c., together with water and its 
elements ; and when these have been employed to build up the 
portions of the body which need renewal, and to furnish the heat 
necessary for vitality, they pass away as useless excreta. But 
upon examining these excreta we find great changes wrought in 
iSxG forms of comhination of their elements, and though, no doubt, 
the vital force brings about what takes place, yet these changes 
are in the truest sense of the word cJiemical. 
Let us confine ourselves to the three substances named as 
forming part of the food, and to the urine only of man, the horse, 
and the cow, to illustrate this existing and progressive decompo- 
sition in the substances of manure-heaps at the time of their for- 
mation. The combinations of the food are broken up by the 
chemical influence of the vital force, and by the time the refuse 
substances are excreted in a fluid form we find some most amazing 
changes. Water (or its elements) has come into play to yield 
oxygen as wanted, and the corresponding quantity of hydrogen 
has been compelled to seek another element for which it could 
exercise affinity, and with which it could form a true chemical 
compound. At the same time the nitrogenized portions of the 
food have yielded to the forces acting in the direction of decom- 
position, and have given up a portion of their nitrogen to seek 
for some element with which it could combine to form a true 
compound. Thus expelled from old combinations, anck both in 
the nascent state when they can exercise a strong affinity, the 
hydrogen and nitrogen meet and form ammonia, a new substance 
existing in its elements in the food, but wholly unknown till decom- 
position liberated together the nitrogen and hydrogen of which it 
is composed. 
The examination of the urine of men or certain animals shows 
the presence of a body called urea or anojnalous cyanate of am- 
monia. This substance is well known to constitute the most 
valuable part of guano. Urea existed in its elements in the 
food or it could not be found in the excrements, but the whole of 
its complicated structure has been built in the animal laboratory 
whence it issued. Its carbon left the matters with which it was 
originally associated to unite itself with nitrogen which had also 
been expelled, and these two formed the gas which chemists call 
cyanogen. This gas united itself with some liberated oxygen 
