AtMOSPIlKRir AIR AS TIIK FOOD OF PLANTS. 7") 



When nitric acid is heated with lean flesh (fibrin), nitric 

 oxide and nitrogen g.ises botli appear. It is thus seen 

 that by successive steps of deoxidation nitric acid may 

 be gradually reduced to nitrous acid, ammonia, nitric oxide, 

 nitrous oxide, and finally to nitrogen. 



Tests for I\iti'ic and Citrous Acids. — The fact that 

 these substances (iften occur in extremely minute quantities renders it 

 needful to employ very delicate tests for their recognition. 



Price's Test. — Free nitrous acid decomposes iodide of potassium in the 

 Bame manner as ozone, and hence gives a blue color, witli a mixture of 

 tliis salt and starch-piiste. Any niti'ite produces the same effect if to 

 tlie mixture dilute sulphuric acid be added to liberate the nitrous acid. 

 Pure nitric acid, if moderately dilute, and dilute solutions of nitrates 

 mixed with dilute sulphuric acid, are without immediate efiFect upon 

 iodide-of-pota-sium-starch-paste. If the solution of a nitrate be min- 

 gled with dilute sulphuric acid, and ai^itated for some time witli zinc 

 filiui^s, reduction to nitrite occurs, and then addition of the starch-paste, 

 etc., gives the blue coloration. Accorditig to Seh6nl)ein, this test, first 

 proposed by Price, will detect nitrous acid when mixed with one-hund- 

 red-tliousand times its Aveight of water. It is of course only applicable 

 in the absence of other oxidizing agents. 



07-een Vitriol Test. — A very characteristic test for nitric and nitrous 

 acids, and a delicate one, though less sensitive than that just describ- 

 ed, is furnished by common gieen vitriol, or protosulpliate of iron. 

 Nitric oxide, the red gas which is evolved from nitric acid or nitrates by 

 mixing them with excess of strong sulphuric acid, and from nitrous acid 

 or nitrites by addition of dilute sulj^hiiric acid, gives with green vitriol a 

 peculiar blackish-brown coloration. To test for minute quantities of 

 nitrous acid, mix the solution with dilute sulphuric acid and cautiously 

 pour this liquid upon an equal bulk of cold saturated solution of green 

 vitriol, so that the former liquid floats upon the latter without mingling 

 much with it. On standing, the coloration will be perceived where the 

 two liquids are in contact. 



Nitric acid is tested as follows: Mix the solution of nitrate with an 

 equal volume of concentrated sulphuric acid ; let the mixture cool, and 

 pour upon it the solution of green vitriol. The coloration will appear 

 between the two liquids. 



Formation of IVitrogen Compounds in the Atmosphere. 



— a. From free nitrogen, by electrical ozone. Schonbein 

 and Meissner have demonstrated that a discharge of elec- 

 tricity through air in its ordinary state of dryness causes 

 oxygen and nitrogen to unite, with the formation of nitric 

 peroxide, NO^. Meissner has proved that not the elec* 



