ESSAY ON IRRIGATION. 73 



carried on much on the same principles, and is perfectly involun- 

 tary in both. 



The indispensable agency of water, in constituting the fluids, 

 and carrying on the circulation in these systems of vessels, has 

 been universally acknowledged ; and could not be overlooked 

 by the most careless observer, while he saw innumerable instan- 

 ces in which plants wither and dry for want of this substance. 

 But while this universal agency has been acknowledged, it is 

 believed that a very inferior office has been assigned to it, from 

 that which it really performs. It has been considered as the 

 mere vehicle which carried and deposited the nutritious particles 

 of other substances, while it in reality was contributing much 

 the largest portion of the actual nourishment to the plants, which 

 annually clothe our earth in living green. 



If this idea is correct, then he who possesses water at his 

 command with which to supply his plants at pleasure, or who 

 has a soil adapted to attract and retain moisture, in suitable 

 quantities, possesses a mine of inexhaustible wealth, from which 

 he can draw at pleasure, in proportion to his industry and his 

 wants. 



In proof of the abstract principle that water constitutes in a 

 very large proportion the food of plants, I may be allowed to 

 mention one or two accurate experiments of distinguished philos- 

 ophers upon the subject, which appear to me to be quite decisive 

 on the case. 



" Mr. Boyle dried in an oven, a quantity of earth proper for 

 vegetation, and after carefully weighing it>, planted in it the seed 

 of a gourd ; he w^atered it with pure rain water, and it produced 

 a plant which weighed fourteen pounds, though the earth pro- 

 ducing it had suffered no sensible diminution." 



" A willow tree was planted by Van Helmont in a pot con- 

 taining a thousand pounds of earth. This plant was watered 

 with distilled water or pure rain water ; and the vessel so cov- 

 ered as to exclude all solid matter. At the end of five years, 

 upon taking out the plant, he found it had increased in weight 

 119 pounds, though the earth had lost only two ounces of its 

 original weight." 



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