LECTURE IV. 
SILICA. 
Besides the raphides, another and more insoluble 
inorganic material, silica, is met with abundantly in 
certain orders of plants, not in a crystalline form, nor 
contained in the interior of cells, like the raphides, but 
diffused generally throughout the structures in which it 
occurs, and this connexion is so intimate and equable, that 
it forms a complete skeleton of the tissues after the soft 
vegetable matters have been destroyed ; in fact, the part 
it plays with reference to the organized tissues in which 
it is deposited is precisely analogous to that existing 
between the animal and earthy elements of shell. Silica 
exists in such great abundance in the cuticle of a plant 
known as Equisetum hyemale , or Dutch rush, that on 
this account the stems are employed by carvers in wood 
and modellers in clay as a substitute for sand-paper. 
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