EARTHS. 47 



plied in very small quantities. If they are administer- 

 ed in great abundance they destroy the plant. 



And the argument against their utility that has been 

 drawn from the small proportion in which they are 

 found to exist in the plant itself, is altogether inadmis- 

 sible ; because it is very well known that some particu- 

 lar ingredient may be essential to the composition of a 

 body, and yet constitute but a very small proportion of 

 its mass. 



5. EARTHS. — As most plants have been found 

 by analysis to contain a portion of alkaline or earthy 

 salts, so most plants have been found to contain also a 

 portion of earths ; and as the two substances are so 

 nearly related, and so foreign in their character to 

 vegetable substances in general, the same enquiry has 

 consequently been made with regard to their origin. 



It seems to have been the opinion of Lampadius that 

 the earths contained in plants are merely the effect 

 of vegetation, and altogether independent of the soil in 

 which they grow ; and extravagant as the opinion is, it 

 has been made to assume the semblance of resting .upon 

 experiment. Saussure has exposed the absurdity of 

 this opinion, and the fallacy of the experiments which 

 led to its adoption ; and it is at present more generally 

 believed that the earths are absorbed with the water in 

 which they are dissolved. 



It is probable, however, that plants are not indebted 

 merely to the soil for the earthy particles which they 

 contain. They may acquire them partly from the at- 

 mosphere. Margray has shown that rain water contains 

 silica in the proportion of a grain to a pound ; which, 

 if it should not reach the root, may possibly be absorb- 

 ed along with the water that adheres to the leaves. 



But although the earths are thus to be regarded as 



