74 SOURCE AND ASSIMILATION 



several wet days occur, the rain of the first must 

 contain more of it than that of the second. The 

 rain of a thunder-storm, after a long protracted 

 drought, ought for this reason to contain the 

 greatest quantity, which is conveyed to the earth 

 at one time. 



But all the analyses of atmospheric air, hitherto 

 made, have failed to demonstrate the presence of am- 

 monia, although according to our view it can never 

 be absent. Is it possible that it 'could have escaped 

 our most delicate and most exact apparatus ? The 

 quantity of nitrogen contained in a cubic foot of 

 air is certainly extremely small, but notwithstand- 

 ing this, the sum of the quantities of nitrogen from 

 thousands and millions of dead animals is more than 

 sufficient to supply all those living at one time with 

 this element. 



From the tension of aqueous vapour at 15 C. 

 (59 F.) = 6,98 lines (Paris measure) and from its 

 known specific gravity at C. (32 F.), it follows 

 that when the temperature of the air is 59 F. and 

 the height of the barometer 28", 1 cubic metre or 

 64 Hessian cubic feet of aqueous vapour are con- 

 tained in 487 cubic metres, or 31,168 cubic feet 

 of air ; 64 cubic feet of aqueous vapour weigh 

 about 1^ Ib. Consequently if we suppose that the 

 air saturated with moisture at 59 F. allows all 

 the water which it contains in the gaseous form 

 to fall as rain ; then 1 Hessian pound of rain- 

 water must be obtained from every 20,800 cubic 



