ON THE AIR AND WATER OF TOWNS. 19 
Organic matter in contact with water constantly gives off an odour of some 
kind, and especially if heated, so that it would appear as if steam or vapour 
were capable of taking up much more than that which we call volatile 
matter. 
: if organic matter be allowed to decompose in the air it gives out carbonic 
acid, ammonia, sulphuretted hydrogen, and probably other gases. Priestley 
has shown that if it decomposes in water it gives out an inflammable gas. 
If however it be exposed to the action of soil, other circumstances being 
ourable, it is converted partly into nitric acid. 
None of these cases occur purely in our towns, but all of them occur to 
‘some extent. Carbonic acid and ammonia occur in all reservoirs of refuse, 
_and sulphuretted hydrogen occurs also in abundance. It was once very 
perceptible in London, as Sir Kenelm Digby complains much of the 
state of the streets, when silver could not be kept clean in his day. This 
“may be observed now in many towns, and is in fact not uncommon. This 
_isa disagreeable smelling gas, and wherever it is abundant will be easily 
; ented by the nose. It may be detected readily in many courts and alleys, 
so at the mouths of sewers, and in some parts of the Irwell and Medlock 
at t Manchester, where they are filled with organic matter and alkaline and 
earthy salts. Ammonia generally accompanies it so as to diminish its bad 
effects. 
_ Ammonia itself is probably of no injury unless in excessive quantities, and 
may be considered as one of the most wholesome forms in which nitrogen 
and hydrogen, as gases, pass into the air. A decomposition such as this occurs 
oO dinarily in towns, as there is a certain exposure to air always. 
In cases where there is no exposure, or at least when the substance is in 
water, inflammable gases are produced, as Priestley has shown and Liebig 
has to some extent explained. It would seem as if, when decomposition 
commenced, oxidation of one portion necessarily took place, leaving the 
‘other portions without oxygen, unless in cases where an abundance 
uld be obtained. Dalton found the gas from the floating island at Der- 
ntwater to contain carburetted hydrogen and nitrogen. The carbon and 
the hydrogen are deprived of oxygen entirely, whilst more oxidized 
bodies, as carbonic acid and humus, are left, the latter body to be in time 
tirely oxidized, as Liebig has shown. Whether the nitrogen comes off 
e@ or as ammonia, the same division of a substance into oxidized and 
idized occurs as we see in the fermentation of sugar, where carbonic 
a body oxidized, and alcohol a body to a great extent deoxidized, occur. 
have only to suppose compounds of carbon, hydrogen and nitrogen, 
ing from decomposing matter, to show us the great danger. It is not to 
rusted that these bodies always appear in the mode of combination men- 
ed here; their modes of combining are various, and these elements 
n the most active poisons known to us. 
certain amount of moisture is almost essential to the escape of odour from 
y bodies; it probably arises from two causes. The vapour of water is 
ehicle for organic matter, and water favours decomposition in bodies, so 
as they decompose the vapour is given out. From whatever cause, it will 
ound that moisture rapidly facilitates the escape of odour. Mineralogists 
‘ail themselves of this when they breathe on a mineral and then ascertain 
ie smell. The moisture of an evening, or even artificial moisture, causes 
flowers to give their scents, and the moist state of the atmosphere betore 
rafter a shower causes also a great fragrance in a flower-garden. But 
hilst this is caused the same laws are operating for injurious effects, 
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