Air is a mixture of gases in the following on the dry
sample
|
Nitrogen
|
By Volume
|
79.0
|
By weight
|
76.8
|
|
Oxygen
|
By Volume
|
20.97
|
By weight
|
23.16
|
|
Carbon Dioxide
|
By Volume
|
0.03
|
By weight
|
0.04
|
Moisture is always present in the atmosphere, but in
varying proportions depending on weather and temperature,
but a rough average of 1% may be regarded as normal.
The breathing of oxygen is absolutely necessary to human
life and for ordinary combustion. The effects of a
decrease in oxygen percentage are as follows:-
|
Oxygen %
|
Nitrogen %
|
Effects
|
|
20.97
|
79.03
|
Breathing and combustion normal
|
|
19
|
81
|
Breathing almost normal, lamp or candle flame
loses 30% luminosity
|
|
17
|
83
|
Breathing almost normal, combustion is
difficult
|
|
15
|
85
|
Breathing nearly normal, flame extinguished
between 17 & 15% Oxygen
|
|
12
|
88
|
Breathing deepens, flame extinguished between
17 & 15% Oxygen
|
|
10
|
90
|
Abnormal shortness of breath, face flushed and
darkened
|
|
5
|
95
|
Panting, unconsciousness and death
|
|
0
|
100
|
Convulsions and death in a very short time
|
It will be readily understood from the above figures
why the Coal Mines Act, 1911, requires that at least 19
per cent. of oxygen shall be present in the atmosphere of
all roads and workings that are in use.
Exemption is made where ventilation is accidentally
interrupted, but no person is to work in less than 19
per cent. of oxygen unless engaged in restoring the
ventilation. Another exemption is granted in the case of
mines liable to spontaneous combustion, but only by order
of and subject to conditions laid down by the Mines
Department.
The gases most commonly found in mines are oxygen,
nitrogen, carbon dioxide, and methane. In connection with
gob fires, blasting operations, and explosions, carbon
monoxide is very common, while in these and cases hydrogen
sulphide, hydrogen, ethylene, nitrous oxide, etc.,
sometimes occur.
The chief noxious gases are the very poisonous carbon
monoxide and carbon dioxide. Hydrogen sulphide is
extremely poisonous, but rarely occurs in dangerous
quantity. These gases are also said to be deleterious, as
they arc more or loss destructive of life.
The Chief Inflammable Gas is methane, which constitutes 97
to 100 % of the inflammable constituent of firedamp.
The Damps
The word " Damp " originally meant "
Mist " or " Vapour," but is now used as
meaning " Gas," and prefixes are attached to
indicate the various gases according to their
peculiarities
Fire-damp is inflammable and consists chiefly of
methane (marsh gas).
Black-damp extinguishes flame and causes death by
suffocation It is composed of carbon dioxide and nitrogen.
It is also known as stythe or choke-damp.
White-damp is a subtle and extremely poisonous gas,
also known as carbon monoxide.
After-damp is the gas resulting from an explosion; it
nearly always contains dangerous amounts of carbon
monoxide.
Stink-damp, or sulphuretted hydrogen, is hydrogen
sulphide and is a very poisonous gas with a pungent smell
of rotten eggs, but it is seldom found in dangerous
quantities.