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dermis of the cane yielded 9 grains of silex. He also found it in 
English reeds and grasses, and in oats, wheat, barley, &c. in a suf- 
ficient quantity to yield glass with the blow-pipe ; a straw being thus 
converted into a fine globule of glass : the epidermis of the equisetum 
hyemale, or Dutch rush, appearing to consist almost entirely of 
silex*. 
Presuming, therefore, that the silicious induration of various 
substances will be allowed to have proceeded from a deposition of 
silex, from its solution in water, I shall, in my next, proceed to 
apply this principle, whilst endeavouring to account for the formation 
of silicized wood. 
Yours, &c. 
LETTER XXXIII. 
PETRIFIED WOOD DIVIDED INTO SILICIOUS, CALCAREOUS, ALU- 
MINOUS, ETC SILICIOUS DIVIDED INTO SILICIZED WOOD AND 
SILICIZED BITUMINOUS WOOD THE LATTER, INTO CALCEDONIC, 
AGATINE, JASPERINE, AND OPALINE. 
For those substances which bear the marks of having originally 
existed as wood, but which now possesses a stony hardness, from the 
introduction of different earthy matters, it seems proper to retain the 
* Nicholson’s Journal, May, 1799. 
