222 ORIGIN OP THE NITRATES. 



regulated at will. Fig. 33 shows the tubes in place and the 

 experiment ready. 



3. Now, if mixed dry nitrogen and oxygen, or even atmospheric 

 air, are subjected to the action of a series of electric sparks, 

 after a few minutes the test-tube is filled with nitrous vapour, 

 but it would need several hours to arrive at the limit of the 

 reaction. This is, moreover, never complete, the spark inversely 

 decomposing nitric peroxide (see p. 198). 



4. If the operation take place in presence of a solution of 

 potash, the acid gases are gradually absorbed and potassium 

 nitrate is finally obtained. This is Cavendish's celebrated 

 experiment (1785). 



5. The combination of nitrogen with oxygen requires the 

 intervention of a foreign energy represented by 21*6 CaL, 

 when the union of nitrogen with oxygen takes place, forming 

 nitric oxide 



N + = NO. 



The latter compound afterwards unites with an excess of 

 oxygen, forming nitric peroxide. 

 The definitive formation, 



N -f 2 = N0 2 gaseous, 



only corresponds to an absorption of 2 '6 Cal. at the ordinary 

 temperature, a quantity which increases to about 7 Cal. 

 towards 200. 



6. It is precisely in virtue of analogous reactions developed 

 in the atmosphere during the passage of forked and sheet 

 lightning that nitric and nitrous acids are formed. These acids 

 appear in rainstorms, partly in the free state and partly as 

 ammonium nitrate or alkaline nitrates, the latter being derived 

 from the dust of the air. For example, Filhol, at Toulouse, 

 obtained per cubic metre of rain, T09 grms. of nitric acid. 

 From the analyses of M. Barral, one hectare of ground at Paris 

 would have received in November, 1852, from the rain, 659 

 grms. of nitrogen in the form of nitric acid. These quantities 

 are considerable, nevertheless the analysis of cultivated plants 

 has shown that they do not suffice to make good the losses of 

 nitrogen taken from the soil by vegetation. 



Fourth Section. Actions of the Silent Discharge at High Tension. 



1. The combination of nitrogen and oxygen with the formation 

 of nitrous compounds is not only produced by the electric 

 spark, but also by the action of the silent discharge, when the 

 electric tension is very great (see the instruments, pp. 226 

 and 230). 



2. This is, again, a condition which occurs in the atmosphere. 

 During the interval of time which precedes the instant when 

 the discharges of lightning, properly so called, trace a certain 



