36 VEGETABLE PHYSIOLOGY. [LESSON 12. 



fluid silica taking their place. None of the structures found fossilized 

 have a tendency to decomposition, and, from the moment they begin 

 to absorb liquid flint, this process is rendered impossible : they can 

 afford, therefore, to wait the development of time, and whenever the 

 circumstances are favorable, the aqueous particles are removed by 

 evaporation, and the solid silica alone remains. 



215. This process is so remarkably slow in nature, that the soft 

 parts of animals are very rarely found fossil ; but they, in common 

 with bones and woods, may be easily converted into flint, artificially 

 and the softer the tissue (brain) the quicker and more certain the 

 experiment. 



LESSON XII. 



SILICA, (CONTINUED.) 



216. Silica obtains to the greatest extent in the lowest forms of 

 vegetable life. A large class of minute (microscopical) organisms, 

 originally classed by Ehrenberg with the animal kingdom, are now 

 known to be vegetable, and from their possessing a double external 

 covering, like the bivalve (bis, two) shells, they are called Diatomacce 

 (two-atoms). 



217. The loricae (shells) of the organic structures included under 

 this head are formed entirely of pure flint, the pattern (so to speak) 

 being always remarkable, peculiar, and limited to species. 



218. The Arachnoidiscus (Spider's web disc), found in Peruvian 

 Guano (Fig. 64), is a very beautiful 



example ; the specimen has been sub- 

 ject to long continued boiling in 

 strong nitric acid a usual method of 

 procuring these forms for microscopical 

 purposes to free them from the ex- 

 traneous materials with which they 

 were surrounded ; the exact, nay, 

 mathematical precision with which it 

 is divided, renders it a very remark- 

 able illustration. 



219. Kindred forms are so abun- 

 dant, both recent, in all the fresh- Arachnoidiscus Ehrenbergii. 



water pools, and fossil, constituting by far the greater part of the 



