194 
Geology of Sydney. 
vegetation. That coal once lived in the form of luxur- 
iant plant life is one of the best established conclu- 
sions of science. The chemist can help us here. He 
tells us that the composition of coal is very similar to 
that of trees and plants. When coal is burnt, an ash 
is left, not different from the ash left on burning 
plants. Good coal contains about 80 to 90 per cent, 
of carbon, while wood contains about 50 per cent, of 
the same element. Woody matter consists of half its 
own weight of carbon — the other half of its weight 
being the gases hydrogen, oxygen, and nitrogen. 
Carbon is, of course, the more stable of these elements, 
and the gases are easily driven off. If wood is heated 
in an open vessel, it burns — that is, its carbon unites 
with the oxygen of the air, giving out, at the same 
time, light and heat, and nothing remains but mineral 
matter, in the shape of ash. But when wood is heated 
in a closed vessel, out of contact wit!) the air, the 
unstable gases are driven off, and the wood becomes 
charcoal or carbon. In a similar way, vegetable 
matter, when in contact with air, disappears by 
decomposing — a process not essentially different from 
burning, decomposition being merely slow combustion. 
When, however, a vast accumulation of vegetable 
matter is shut out from the air, and therefore from 
free oxygen, pressure or chemical action tends to drive 
oft 1 the oxygen and hydrogen constituents, and we 
have a substance approaching the condition of carbon 
and coal. 
Wood. Peat. Lignite. Coal. Anthracite. 
Carbon, per cent.... 50 CO 66 81 91 
