326 Chemical Clianges in the Fermentation of Dung. 
and formed a fluid called cjanic acid, which in its turn sought 
and found a suitable substance wherewith to combine in the free 
gas called ammonia ; and thus results a solid — urea — which when 
voided from the animal possesses no apparent property in com- 
mon with the food taken in the first place, nor with the other 
bodies into which it is resolved almost as soon as ejected. It is 
itself a crystalline colourless solid, but in the presence of water 
soon becomes resolved into the two gases carbonic acid and 
ammonia. 
This series of changes might be carried to a great length, but 
enough has probably been said to show that at the very time that 
animal excreta leave the body they are in a state of change, and 
consequently that what takes place in a manure heap is actually 
initiated as soon as its most valuable constituents become subject 
to our observation. 
The same remarks apply to the vegetable portions. Water 
and a temperature above the freezing point instantly originate a 
series of changes which would end in the entire disruption of the 
substance of straw or other similar matter and bring forth a new 
arrangement of their elements, and these conditions are obviously 
present the moment litter is thrown down in a farm yard or as 
bedding for cattle. 
We must therefore regard the newly formed heap as being in 
a state of change when first made, and endeavour to follow out 
those changes which occur subsequently by the light of the 
knowledge we already possess. 
The decay of substances placed together in a manure heap 
conforms to the laws which regulate t]ie combustion of similar 
bodies. Putting out of sight for the present the inorganic con- 
stituents we have a quantity of matter consisting principally of 
carbon, oxygen, and hydrogen, variously grouped to form the 
different kinds of vegetable tissue composing the straw and that 
which is voided unaltered in the animal excrements. The 
oxygen and hydrogen in vegetable tissue are always found in the 
same proportion as in water, that is eight parts by weight of 
oxygen to one part by weight of hydrogen. 
When a rotten apple is mixed with a quantity of sound ones 
the parts of the latter which are in contact with tlie former are 
altered in condition and begin to decay. This well known fact 
is the reason for constantly examining kept fruits and removing 
those which show any symptom of rottenness. It is further 
known that when once an apple is affected the entire substance 
soon undergoes decomposition. Thus the putrefaction of manure 
is begun and propagated. Substances which are themselves in 
a state of putrefaction (dung of animals) are brought into contact 
with others which are not (straw, &c.), and communicate by that 
