206 OPEN AIR GKAPE CULTUEE. 



down roots deep into the subsoil if it is open and 

 porous. 



Rain is another important source not only of am- 

 monia and gases, but of mineral matter. We quote the 

 following from Lindley's "Theory of Horticulture:" 



" The researches of chemists have shown that all rain 

 water contains ammonia, a compound of hydrogen 

 and nitrogen, and thus the source of the nitrogen 

 absorbed by plants was explained. But it has also 

 been shown, especially by M. Barral, that other sub- 

 stances upon which plants feed are contained in rain 

 water to a much greater amount than was suspected. 

 This observer was led, during six months of 1851, to 

 examine minutely the water collected in the rain 

 gauges of the Observatory of Paris. ' His mode of 

 investigation is declared by Messrs. Dumas, Bous- 

 singault, Gasparin, Regnault, and Arago, names fore- 

 most in French science, to be free from all objection, 

 and to bear the most counter trials to which they 

 could expose it. M. Barral states, that although the 

 quantities of the following substances varied in dif- 

 ferent months, yet the monthly average from July to 

 December, inclusive, was as follows : 



"substances in a cubic metre of rain water. 



ORAMMES. 0RAIK3. 



Nitrogen, 8.36 = 129. 



Nitric Acid, 19.09 = 294. 



