38 THE VITAL FUNCTIONS. 



An example occurs in the case of gold, the ductility 

 of which is totally destroyed by the presence of a 

 quantity of either antimony or lead, so minute as 

 barely to amount to the two thousandth part of the 

 mass ; and even the fumes of antimony, when in 

 the neighbourhood of melted gold, have the power 

 of destroying its ductility. In the experiments 

 made by Sir John Herschel on some remarkable 

 motions excited in fluid conductors by the trans- 

 mission of electric currents, it was found that minute 

 portions of calcareous matter, in some instances less 

 than the millionth part of the whole compound, are 

 sufficient to communicate sensible mechanical mo- 

 tions, 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 exhalation of the water 

 which held it dissolved ; and it is found, accord- 

 ingly, that the greater portion of this silica is 

 actually deposited in the leaved, and the parts 

 adjacent to them. When once deposited, it seems 

 incapable of being again taken up, transferred to 

 other parts, or ejected from the system ; and hence, 

 in course of time, a considerable accumulation of 

 siliceous particles takes place, and by clogging up 

 the cells and vessels of the plant, tends more and 

 more to obstruct the passage of nourishment into 

 these organs. This change has been assigned as a 

 principal cause of the decay and ultimate destruc- 



* Philosophical Transactions for 1824, p. 162. 



