ON THE MEPICAL TOrOGRAI'IlY OF SINGAPORE. 
471 
another proportion of carbon to form earburetted hydrogen. In a 
wet marsh this decomposition goes on with redoubled vigour com¬ 
pared with a dry one, from the water furnishing abundance of oxygen 
and hydrogen to form carbonic acid and earburetted hydrogen. 
One advantage of draining consists in carrying off this superabun¬ 
dant water and reducing the carbon to combine with the water sole¬ 
ly contained in the vegetable, by which the formation of carburet- 
ted hydrogen is almost entirely prevented, Liebig states, “ when 
wood putrefies in marshes, carbon and oxygen are separated from 
its elements in the form of carbonic acid, and hydrogen in the form 
of earburetted hydrogen. But when wood decays or putrifies in 
the air, its hydrogen does not combine with carbon, but with oxygen 
for which it has a much greater affinity at common temperatures.” 
In a marsh we may have the two processes going on at the same 
time, towards the surface we may have the decomposition of vege¬ 
table matter and the formation of earburetted hydrogen, while be¬ 
low we may have the slow combination of the carbon with oxygen 
forming carbonic acid, an act of slow combustion by which the wood 
is charred : how often planters here must have noticed that charring 
of wood ! 
Yet another formation is seen in our rivers and its tributary ca¬ 
nals when under tidal influence, when carbon unites with oxygen 
and hydrogen to form carbonic acid, and earburetted hydrogen, ano¬ 
ther proportion of hydrogen unites with the sulphur of the sul¬ 
phates of the salt water to form sulphuretted hydrogen. Here there 
is an addition to the product of fresh water marshes in the exist¬ 
ence of sulphuretted hydrogen, and in no small quantity. In some 
fresh water marshes sulphuretted hydrogen may be generated in 
small quantities, when we have a particular class of plants, as the cru- 
ciferte ; but if these plants are buried deep, so that decomposition 
goes on but slowly, there we have sulphurets formed, in the shape 
of pyrites or crystallized sulphuret of iron. Regarding the addition 
of sulphuretted hydrogen to the other products found in fresh water 
swamps, let the reader pay particular attention, as it will go far to 
elucidate the nature of miasm. 
