LECT. III.] EPIDERMIS. 93 



in alcohol, and lastly in ether, every thing soluble 

 in it will be extracted by these liquids, and the 

 insoluble part which remains be found to be com- 

 posed of interlaced fibres, easily subdivided and 

 having- some degree of transparency : these are 

 the ligneous fibres. They have neither taste nor 

 odour, and remain unaltered by exposure to the 

 atmosphere : but although insoluble in water, 

 alcohol, or ether, the fixed alkalies and mineral 

 acids dissolve and decompose ligneous fibre. The 

 relative quantity of this fibre in any plant may be 

 pretty accurately ascertained, by exposing a given 

 quantity of the wood to a moderate fire, in close 

 vessels, for a number of hours sufficient to convert 

 it into charcoal ; for, as the wood only becomes 

 charcoal and the other parts are dissipated, the 

 proportional weight of the charcoal obtained shows 

 the quantity of the ligneous fibre contained in the 

 wood. By experiments of this kind, carefully per- 

 formed on the wood of the Poplar, the Lime, the 

 Fir, the Maple, the Elm and the Oak, Count 

 Rumford ascertained that the quantity of ligneous 

 fibre in each of these trees was equal to nearly 

 nine twentieths of their wood in its natural 

 state *. 



The Epidermis is that portion of the vege- 

 table structure which is exterior to all the others ; 



* Gilberts Annalen der Physick, - xiv. p. 25; and Thomson's 

 System of Chemistry, 5th edit. vol. iv. p. 186. 



