216 
COLLIEKY EXPLOSIONS* 
dreaded as much or efen more than the explosion. Miners may 
escape the latter, but unless immediately rescued, or able to 
obtain fresh air, death is certain. 
Carbonic oxide gas differs from the two' previous gases ; 
though in chemical composition it is similar to the carbonic acid, 
with the exception of having one atom of oxygen the less. It is 
a combustible gas, burning -^ith a blue flame, but does not readily 
explode when mixed with air. When inhaled, carbonic oxide 
combines with the hgemaglobin of the blood and becomes highly 
poisonous ; even in very small quantities it produces a headache 
and giddiness. 
I now pass on to consider the ventilation of collieries, upon 
which the safety of mines so much depends. 
Suppose I take a glass cylinder (the deeper the better), and 
divide it into tw’O partitions by placing a piece of cardboard down 
the centre, hut leaving a space at the bottom of the cylinder of 
about an inch. Next, suspend in one of the divisions a lighted 
candle or lamp ; the result will he that a column of air will 
ascend the partition in which the candle is placed, while at the 
same time a column of cold air will descend the other division of 
the cylinder. This is because the air has become rarefied (in the 
division in which the candle is placed) by reason of the heat ; or, 
in other words, the air has become lighter in the one division 
than in the other, by reason of expansion due to the heat, and 
therefore rises, while the heavier column rushes to fill up the 
vacuum that w^ould otherwise be created. Thus a constant 
current is established so long as the light burns. The rate at 
which this current moves will depend upon the difference in 
weight of the two air columns, and this again will be regulated 
by the temperature. 
Now, it is upon this principle that ventilation by natural 
draught is established in collieries. The partition of the 
cylinder in which I represented the candle as burning may be 
