Chemical Changes in the Fermentation of Dung. 327 
contact their own state of change, and hence the action is pro- 
pagated from one particle of matter to another throughout the 
heap. 
In every instance of this kind one laio is oleyed, and we think 
that law may be made clear lo the reader though he be not a 
chemist. It is to be borne in mind that the substances to be 
changed are composed chiefly of carbon, oxygen, and hydrogen, 
of which the last clement is incomparably the most prone to de- 
composition (most combustible).* Wherefore as soon as a decom- 
posing nitrogenized substance communicates to a sound one the 
state of change (motion) in which its own elements are placed, 
the elements of that other bod}- begin to be liberated, and the 
freed hydrogen seizes with avidity upon any oxygen in its neigh* 
bourhood and forms water. The liberation of one element is the 
liberation of others, and the carbon now seeks oxygen whereto it 
may be allied to form carbonic acid. But till the hydrogen has 
become satiated with oxygen, that is, till one of hydrogen by 
weight has united with eight of oxygen by weight, no oxygen can 
unite with carbon, or in other words no (oxide of carbon) carbonic 
acid can be formed. 
This is well illustrated by what takes place when vegetable 
fibre undergoes decomposition with an insufficient supply of 
oxygen. Straw or wood rotting under water so change that part 
of the hydrogen combines with the carbon and forms light car- 
buretted hydrogen (marsh gas), Avhereas under ordinary circum- 
stances with free access of air the carbon unites with oxygen 
and forms carbonic acid. The firedamp of coal-pits is similarly 
formed by a union of hydrogen with the carbon of the decaying 
vegetable fibre of the coal. Again when wood or coal (which 
consist chiefly of vegetable fibre of identical composition with 
that of straw) are distilled in close vessels, where the supply of 
oxygen is of course limited to that contained in their own sub- 
stances, water is formed as long as oxygen is supplied in sufficient 
quantity, and the liberated carbon has no alternative but to com- 
bine with hydrogen and form illuminating gas. Similarly, when 
a candle (which consists like vegetable fibre of carbon, hydrogen, 
and oxygen) burns, the hydrogen may be seen burning first in the 
blue portion at the bottom of the flame, where it forms water 
with atmospheric oxygen, while the carbon passes upwards into 
the white portion unable to combine with oxygen (to become 
burnt) until the hydrogen has been satiated. 
This strong affinity of hydrogen for oxygen leads to the forma- 
tion of a large quantity of water during the fermentation of dung 
* By this is meant, that -when an existing compound containing hydrogen is 
broken up, the hydrogen has the strongest tendency to enter into ue-w combinations. 
