ROOTS FEED ON WATEE, ETC. 29 



He did not ascertain whether all these substances are contained in 

 rain-water collected at a distance from towns. But Dr. Benoe Jones 

 found at least nitric acid in raia- water collected in London, at Kingston 

 in Surrey, at Melbury in Dorsetshire, and far from any town at 

 Clonakelty in Ireland. If we assume that M. Barral's averages 

 represent what occurs on an English acre, the quantity of such 

 substances deposited on that extent of ground may be safely estimated 

 as foUows : — 



The average depth of rain which falls in the neighbourhood of 

 London is well ascertained to be about twenty-four inches per annum. 

 This is at the rate of 87,120 cubic feet, or 2466 cubic metres of rain- 

 water per acre ; and this, according to the proportions per cubic metre 

 in the preceding table, would afford annually of— 



Nitrogen 45^ lbs. 



Nitric acid 103 ,, 



Ammonia ....... 194 ,, 



Chlorine 12^,, 



Lime 35 ,, 



Magnesia 11 i, 



Annual total per acre 227 



Of these substances the three first are of the utmost importance, on 

 account of their entering so largely into the indispensable constituents 

 of the food by which vegetable life is sustained. The quantity of 

 ammonia thus ascertained to exist is about what is expected ia two 

 hundred weight of Peruvian guano ; and bountiful nature gives us, 

 moreover, nearly one hundred and fifty pounds of nitrogenous matter, 

 equally suited to the nutrition of our crops. 



It has been confidently asserted that in addition to their 

 feeding properties, roots are the organs by which plants rid 

 themselves of the secreted matter which is either superfluous 

 or deleterious to them. If you place a plant of . Succory in 

 .water, it will be found that the roots will, by degrees, render 

 the water bitter, as if opium had been mixed with it ; a Spurge 

 will render it acrid; and a leguminous plant mucilaginous. 

 And, if you poison one half of the roots of any plant, the other 

 half will throw the poison off again from the system. Hence it 

 has been thought to follow that, if roots are so circumstanced 

 that they cannot constantly advance into fresh soil, they will 

 by degrees be surrounded by their own excrementitious 

 secretions. More correct experiments have however shown 



