LESSON 11.] SILICA. 35 



but is usually associated with an acid, sometimes oxalic, at other 

 times phosphoric, or carbonic acid. 



205. Silica, on the contrary, is found in large quantity in vegeta- 

 ble tissues in all its integrity as pure flint. 



206. There can be no doubt that the roots obtain this mineral 

 from the earth ; elaborate, or digest it, and reproduce it in a new and 

 characteristic form. 



207. Each of the grasses, for example, affords a large supply of 

 this mineral ; but the/orw and arrangement of it is always peculiar, 

 and such as can only be found in that particular species, in which 

 such form, however, is constant. It may excite our wonder and sur- 

 prise to learn how it is possible for the roots of a plant to decompose, 

 that they may feed upon such an intractable substance as flint ! 



208. Yet the process is a very simple and natural one : in addi- 

 tion to flinty materials, the earth contains large supplies of alkalies 

 potash and soda. 



209. Silica, that resists the action of the most powerful acids, 

 succumbs to alkalies. By union with these elements, the flint is 

 dissolved to a fluid state, and forms the silicate of potash, or of soda, 

 as the case may be. 



210. The Chemist can form artificial silicates : by a process of 

 manipulation of one kind, he forms a silicate soluble in water ; by 

 another process he makes an insoluble silicate. 



211. By their effects it would appear that the silicates formed by 

 nature, are (originally) soluble / that they are so much reduced by 

 the addition of water as to become easy of access to the roots of the 

 plants which absorb them ; at the same time it is possible that the 

 superabundance of the alkaline materials, may render an insoluble sili- 

 cate sufficiently fluid to be appropriated by the roots. 



212. Under any circumstances it is quite certain that the silica 

 found in plants after they have elaborated it, is always perfectly 

 insoluble. If, however, it be formed as a soluble silicate, then it 

 becomes insoluble in the tissues of the plant due, doubtless, to the 

 vital energies of the organism. 



213. The mode by which the tissues of plants and animals become 

 silicifled, or fossilized, is a very simple one : they lie beneath the sur- 

 face of the earth, in and surrounded by the silicates of potash, or soda ; 

 these materials are gradually absorbed by the tissues, which, whether 

 wood or bone, are easily accessible to the transmission of fluids. 



214. By slow and insensible degrees, through a long series of 

 years, the character of the original tissues gradually changes, the 



