34 AGRICULTURE. 



enables them to receive their food, digest and as- 

 similate it to their own substance, reproduce their 

 species, and maintain an existence of longer or 

 shorter duration. Thus far the learned are agreed, 

 but at the next step they differ. 



What is this food that gives to plants their devel- 

 opment, and maturity, and powers of reproduc- 

 tion 1 Lord Bacon believed that water was the 

 source of vegetable life, and that the earth was 

 merely its habitation, serving to keep plants up- 

 right, and to guard them against the extremes of 

 heat and cold. Tull, on the other hand (and, after 

 him, Du Hamel) pronounced pulverized earth the 

 only pabulum of plants, and on this opinion built his 

 system of husbandry. Van Helmont and Boyle op- 

 posed this doctrine by experiments : the former 

 planted and reared a cutting of willow in a bed of 

 dry earth, carefully weighed, and protected against 

 accretion by a tin plate, so perforated as to admit 

 only rain and distilled water, with which it was oc- 

 casionally moistened. At the end of five years the 

 plant was found to have increased one hundred and 

 sixty-four pounds, and the bed of earth to have lost, 

 of its original weight, only two ounces. Boyle pur- 

 sued a similar process with gourds, and with a sim- 

 ilar result. Notwithstanding the apparent conclu- 

 siveness of these experiments, their authority was 

 shaken, if not subverted, by others made by Mar- 

 graff, Bergman, Hales, Kirwan, &c., &c. The first 

 of these showed that the rain water employed by Van 

 Helmont was itself charged with saline and other 

 earthy matter ; Bergman demonstrated this by anal- 

 ysis, while Kirwan and Hales proved that the earth, 

 in which the willow cutting was planted, could ab- 

 sorb these matters through the pores of the wooden 

 box which contained it, and that a glass case could 

 alone have prevented such absorption. Hunter, 

 finding that oil and salt entered into the composi- 

 tion of plants, concluded that these formed their 



