112 ON THE FOOD OF PLANTS. 



described by all, but as to the manner, or through 

 what particular medium, it is supplied and con- 

 sumed, these authors vary in their opinions ; 

 and I cannot but think. Sir Humphry Davy 

 has taken a very mistaken view of this part of 

 his subject. To show that there are other means 

 in nature by which carbonic acid is consumed, 

 than by the leaves of plants, we need only take 

 his own description of the nature and proper- 

 ties of lime. On this part of the subject he 

 says, *' Slacked lime was used by the Romans 

 " for manuring the soil in which fruit trees grew; 

 " this we are informed by Pliny. Marie had been 

 ** employed by the Britons and the Gauls, from 

 '* the earliest times, as a top dressing for land. 

 "But the precise period in which burnt lime first 

 " came into general use, in the cultivation of 

 *' land, is, I believe, unknown : the origin of 

 " the application, from the early practices, is 

 " sufficiently obvious. A substance which had 

 " been used with success in gardening, must 

 " have been soon tried in farming ; and in coun- 

 " tries where marie was not to be found, cal- 

 " cined lime-stone would be naturally employed 

 « as a substitute." 



" The elder writers on agriculture had no 

 *' correct notion of the nature of lime, lime- 

 " stone, and marie, or of their effects, and this 



