all Mineral Substances. 167 
the squeeze in closing up the hollows in a 
much shorter time. The whole area of the 
roof strata in amine will sometimes sink down 
at once, but in general only parts of it sink at 
atime. In the last instance, the air is forced 
out of the part affected, and if it be mixed 
with inflammable air, as it often is in fiery 
coaleries, the most fatal accidents happen to 
the miners then inthe pit; for the mixture is 
sure to reach some of their candles, and to 
produce a terrible explosion. 
