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on their surface is due to a thin coating of silica, 
the same earth of winch quartz-rock and flint are com- 
posed ; and, if two canes which belong to the Grass 
family are rubhed together in the dark, they émit a 
flash of light; and, when a stack of corn or hay is 
burnt, a glassy scoria is often found among the ashes, 
which has originated from the fusion of the silica, 
assisted by the potash in the plant. 
