26 SIXTH RKPORT — 1836. 



Its existence in the epidermis of most monocotyledonous 

 plants proves, that it must be held in solution by the descend- 

 ing sap ; and the latter, in whatever way it may l3e supposed to 

 be elaborated within the texture of the plant, can only obtain its 

 earthy principles fi'om the water which happens to encircle the 

 roots. 

 How farex- On the fact of its solution in water, Turner has lately made 

 plained. gome observations in his Lecture on the Chemistry of Geology*. 



He has shown, that water must have the property of dissolv- 

 ing silica, by contrasting the chemical composition of felspar 

 with that of the porcelain clay which results from its decompo- 

 sition. 



The former, as he represents it, consists of one atom of trisi- 

 licate of potass, with one atom of silicate of alumina, in the pro- 

 portion of nine of silica to one of alumina; whilst porcelain 

 clay consists of seven atoms of silica to two of alumina, or as 

 three and a half to onef . 



Hence water had carried off in some way all the potass, and 

 eight and a half out of twelve proportioijals of the silica, leaving 

 all the alumina and the remainder of the silica untouched. 



Now the solution of the silica may be referred in general to 

 its being exposed, at the moment of its disengagement from its 

 existing combination, to the joint action of water and alkali. 



But it seems to admit of question, whether the latter be 

 realljf essential to the process. 



I have myself found a coating of a substance like hyalite in 

 the fissures of a rock in the island of Ischia, through which hot 

 vapours were constantly issuing, and am at a loss to refer 

 it to any other cause, except the gradual solution of silica in the 

 first instance by the steam, and its precipitation afterwards 

 from it. 



I have also found, in a soft state, coating fissures in a tra- 

 chytic rock, near Schemnitz in Hungary, what appeared to be 

 silex hardening into the condition of hyalite, a mineral occur- 

 ring in many places near, — an ol)servation in which I find myself 

 to be anticipated by M. Beudant. 



Dr Wollaston indeed had observed, and Dr. Turner confirms 



• Phil. Magazine, 1833, vol. iii. p. 20. 

 ■f- Represented symbolically thus : 



(k -f 3 Si) + (.M + 9 Si) Felspar, 



(Al + 3-i^ Si) Porcelain Clay ; 

 so that (K ■\- 3 Si) + 5i Si have been removed, and only 3 J- Si remaios 



