The retorts in which the coal is carbonized are almostluniversally made of fire-clay, and in all but small country works the old singleended retort, which was about 9 ft.
It has been found useful in some cases to examine microscopically the thin film of coal that often covers the pinnae of fossil fronds, in order to determine the form of the epidermal cells which may be preserved in the carbonized cuticle; rectilinear epidermal cell-walls are usually considered characteristic of Cycads, while cells with undulating walls are more likely to belong to Ferns.
It reacts most energetically with many organic compounds, removing the elements of water in many cases and leaving a carbonized mass.
It is represented at the bottom of the lake by a layer of charcoal mixed with implements of stone and bone and other relics highly carbonized.
The disadvantage in this process is that the by-products, such as pyroligneous acid, acetone, wood spirit, &c., are lost; as an alternative method, wood is frequently carbonized in ovens or retorts and the volatile products are condensed and utilized.
