FERNS, FOSSILS AND FUEL 
The sections are cut as thin as the hardness or brittle¬ 
ness of the material will permit. Then they are polished 
on one side and cemented on a piece of glass. The 
exposed side of each section is put on a grinding disk 
and ground with carborundum until it is so thin that 
it is transparent. Another piece of glass is usually 
cemented on top to protect it before it is examined under 
the microscope. The cement used for this purpose is 
Canada balsam or a similar resin. 
This method of preparing coal balls for examination 
of the plant remains in them is extremely laborious and 
naturally requires a long time. Recently a much quicker 
process has been devised, which is now widely used in 
laboratories. The surface of the thin section sliced from 
the coal ball is smoothed and polished, then exposed to 
diluted hydrochloric acid. The acid eats away a thin 
layer of the lime which saturates the petrefact, and the 
cell structure and woody fibers of the plants stick up like 
the fine hairs of velvet. In petrefacts saturated with 
silica, hydrofluoric acid is used instead of hydrochloric 
for this etching. 
After the acid has been rinsed off with water and the 
surface dried, a thin solution of cellulose is spread over 
it, which soon forms a fine, perfectly transparent film, 
in which all the delicate organic particles left sticking 
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