Following the first report dealing with the softening behaviours of coal in carbonization, the results of the same study carried on the four different ranked foreign coals are here commented; especially the several characteristic pyrolyse-changes of coal grains, well observed under the microscope, are mentioned in detail.
The temperatures, at which the first bulb appeares in vitrinite grain, or the same grains become plastic, resolid and anisotropic, are not always dependent upon the coalification-grade; some optical change occures according to the essential nature of coal (the chemical structure), the other rather to the atmosphere in carbonization process.
Thus, the behaviours of coal grains during carbonization are so much complicated that they are not sufficiently known only by chemical way, but the optical mean is indispensable to trace them.
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