126 TECHNOLOGY, 
7. VENTILATION oF Mrnzs. 
The air in mines becomes unfit for respiration either by the consumption of 
oxygen by the miners and lights, or by the development of gases which are 
dangerous or at least do not sustain life, as carbonic acid, hydrogen, sul- 
phuretted and carburetted hydrogen, carbonic oxide, sulphurous acid, and 
the fumes of mercury and arsenic. When there is no reason to apprehend the 
presence of inflammable gas in a mine, the condition of the air is readily 
investigated by lowering down a burning candle, which will only burn in 
respirable air; the presence of sulphuretted hydrogen is recognised by the 
smell, or by the blackening of strips of paper dipped in a solution of 
acetate of lead. When carburetted hydrogen, called jire-damp, is suspected, 
a safety-lamp is lowered, the wire cylinder of which will become entirely 
filled with flame when the air contains one sixth of the gas; when one half 
of the volume of the air is composed of carburetted hydrogen the lamp will 
go out. The practice of setting fire to the inflammable air in mines, which 
was formerly often resorted to, is very objectionable, being not only danger- 
ous to the workmen engaged in doing so, but also to the mines which are 
set on fire, besides having the disadvantage that two volumes of oxygen 
are consumed for each volume of inflammable gas. 
The only effectual way of purifying the air in mines is by the copious 
introduction of pure atmospheric air, a continuous current of it being made 
to enter the mines at one point, and passing out at another after circulating 
throughout the excavations. This ventilation may either be produced solely 
by the difference in gravity of the external air and that in the mines, aided 
by a judicious arrangement of the excavations, when it is called natural 
ventilation ; or else it is caused in part or entirely by machinery, producing 
artificial ventilation. 
1. Narurat Ventization. In mines with but one surface opening, whether 
a shaft or a gallery, the ventilation is very much assisted by large dimensions 
which allow two opposite currents of air to be formed without interfering 
much with each other. In shafts the dripping of water at the sides pro- 
motes a downward current of air, while an upward current takes place in 
the centre. By dividing a shaft or gallery into two parts by a closely fitted 
partition, the ventilation is much augmented; one of the parts may be 
advantageously connected with an air-chimney. The wind may also be 
temporarily made use of by means of a windsail. 
The natural ventilation generally exists in a sufficient degree in mines 
with two surface openings, between which there is a considerable difference 
of level, which may be increased by erecting an air-chimney over the higher 
one. In winter the currents of air are often inconveniently strong, and are 
therefore diminished by doors which partially shut off the draught. 
9. ArtirictaL Ventination. As the natural ventilation depends on the 
difference of temperature of the exterior and that in the mines, it may be 
effectually assisted when it is found insufficient by a furnace placed at the 
bottom of a shaft, by means of which a brisk draught is created. 
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