ATMOSPHERIC AIR AS THE FOOD OF PLANTS. 45 
When nitric acid is heated with lean flesh (fibrin), nitric. 
oxide and nitrogen gases both 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 Nitric and Nitrous Acids, — The fact that 
these substances often occur in extremely minute quantities renders it 
needful to employ very delicate tests for their recognition. 
Price's Test.—F ree nitrous acid decomposes iodide of potassium in the 
same manner as ozone, and hence gives a blue color, with a mixture of 
this salt and starch-paste: Any nitrite produces the same effect if to 
the 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 effect upon 
iodide-of-potassium-starch-paste. If the solution of a nitrate be min- 
gled with dilute sulphuric acid, and agitated for some time with zine 
filings, reduction to nitrite occurs, and then addition of the starch-paste, 
ete., gives the blue coloration. According to Schénbein, this test, first 
proposed by Price, will detect nitrous acid when mixed with one-hund- 
red-thousand times its weight of water. It is of course only applicable 
in the absence of other oxidizing agents. 
Green 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 green vitriol, or protosulphate 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 sulphuric 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 Nitrogen Compounds in the Atmosphere. 
—a. From free nitrogen, by electrical ozone. Schénbein 
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- 
