44 THE VITAL FUNCTIONS. 



Sir John Herschel on some remarkable motions 

 excited in fluid conductors by the transmission 

 of electric currents, it was found that minute 

 portions of calcareous matter, in some instances 

 less than the millionth part of the whole com- 

 pound, are sufficient to communicate sensible 

 mechanical motions, and definite properties to 

 the bodies with which they are mixed.* 



As Silica is among the densest and least soluble 

 of the earths, we might naturally expect that 

 any quantity of it taken into the vegetable 

 system in a state of solution, would very early 

 be precipitated from the sap, after the exhala- 

 tion of the water which held it dissolved ; and it 

 is found, accordingly, that the greater portion of 

 this silica is actually deposited in the leaves, 

 and the parts adjacent to them. When once 

 deposited, it seems incapable of being again 

 taken up, and transferred to other parts, or 

 ejected from the system : and hence, in course 

 of time, a considerable accumulation of silicious 

 particles takes place, and by clogging up their 

 cells and vessels, tends more and more to ob- 

 struct the passage of nourishment into these 

 organs. This change has been assigned as a 

 principal cause of the decay and uhimate des- 

 truction of the leaves : their foot-stalks, more 

 especially suffering from this obstruction, perish, 



* Philosophical Transactions for 1824, p. 162. 



