ON THE COMPOSITION OF SOILS, &C. 191 



" gillaceous earth may also be finely difilised, so 

 " as to pass through the best of filtres ; so may 

 " also calx, as appears from the quantity Mar- 

 " graaf found in the purest rain water." 



On this part of the subject, after reciting a 

 great number of experiments, Sir Humphry 

 Davy observes, " The general results of this 

 " experiment, are very much opposed to the 

 " idea of the composition of the earths by plants, 

 " from any of the elements found in the atmo- 

 " sphere or the water." 



He also says, " As the evidence on the sub- 

 "je.ct now stands, it seems fair to conclude, 

 " that the different earths and saline substances, 

 " found in the organs of plants, are supplied 

 * by the soils in which they grow, and in no 

 " cases composed by new arrangements of the 

 " elements in air or water." 



Here again is a great difference in the opinion 

 of those great chemists, and on a most import- 

 ant point; it being admitted that earths are 

 a necessary ingredient in the composition of 

 plants, the question naturally arises, how, and 

 in what proportions, and in what state, is it re- 

 quired by vegetables ? If Sir Humphry Davy's 

 conclusions are just, a minute examination or 

 analysis of soils, would be requisite to form a 

 correct judgment of its capacity ; and to these 



