HORSETAILS. 219 



substance may be destroyed by maceration, the form of the 

 plant being preserved entire in the flinty coating. It has 

 been found that the ashes contain half their weight of silica. 

 Some very interesting observations of Dr. Brewster, on the 

 microscopic structure of this siliceous coating in K hyemale, 

 first published by Dr. Greville, we may quote. 



" On subjecting a portion of the cuticle to the analysis of 

 polarized light under a high magnifying power/' writes Dr. 

 Greville, " Dr. Brewster detected a beautiful arrangement 

 of the siliceous particles, which are distributed in two lines 

 parallel to the axis of the stem, and extending over the 

 whole surface. The greater number of the particles form 

 simple straight lines, but the rest are grouped into oval 

 forms, connected together like the jewels of a necklace by a 

 chain of particles forming a sort of curvilinear quadrangle ; 

 these rows of oval combinations being arranged in pairs. 

 Many of those particles which form the straight lines, do 

 not exceed the five-hundredth part of an inch in diameter. 

 Dr. Brewster also observed the remarkable fact, that each 

 particle has a regular axis of double refraction. In the straw 

 and chaff of wheat, barley, oats, and rye, he noticed analo- 

 gous phenomena, but the particles were arranged in a dif- 

 ferent manner and displayed figures of singular beauty. 



