'88 THE FAIRY-LAND OF SCIENCE. 
Now the pressure which we can exert by machinery 
is absolutely nothing compared to the weight of all 
those hundreds of feet of solid rock which lie over the 
coal-beds, and which has pressed them down for thou- 
sands and perhaps millions of years ; and besides this, 
we know that parts of the inside of the earth are very 
hot, and many of the rocks in which coal is found are 
altered by heat. So we can picture to ourselves that 
the coal was not only squeezed into a solid mass, 
but often much of the oil and gas which were in the 
leaves of the plants was driven out by heat, and the 
whole baked, as it were, into one substance. The dif- 
ference between coal which flames and coal which 
burns only with a red heat, is chiefly that one has been 
baked and crushed more than the other. Coal which 
flames has still got in it the tar and the gas and the 
oils which the plant stored up in its leaves, and these 
when they escape again give back the sunbeams in a 
bright flame. The hard stone coal, on the contrary, 
has lost a great part of these oils, and only carbon 
remains, which seizes hold of the oxygen of the air 
and burns without flame. Coke is pure carbon, which 
we make artificially by driving out the oils and gases 
from coal, and the gas we burn is part of what is 
driven out. 
We can easily make coal-gas here in this room. I 
have brought a tobacco-pipe, the bowl of which is 
filled with a little powdered coal, and the broad end 
cemented up with common clay. When we place 
this bowl over a spirit-lamp and make it very hot, the 
gas is driven out at the narrow end of the pipe and 
lights easily (see Fig. 53). This is the way all our gas 
