78 HOW CROPS FEED. 
When the combustion is so perfect, that the resulting wa- 
ter is colorless and pure, only nitrous acid is formed ; 
when, on the other hand, a trace of organic matters es- 
capes oxidation, less or no nitrous acid, but in its place 
ammonia, appears in the water, and this under circum- 
stances that preclude its absorption from the atmosphere. 
Zabelin gives no proof that the combustibles he eme- 
ployed were absolutely free from compounds of nitrogen, 
but otherwise, his experiments are not open to criticism. 
Meissner’s observations were indeed made under some- 
what different conditions; but his negative results were 
not improbably arrived at simply because he employed a 
much less delicate test for nitrous acid than was used by 
Schénbein, Boettger, Jones, and Zabelin.* 
We must conclude, then, that nitrous acid and ammonia 
are usually formed from atmospheric nitrogen during rap- 
id combustion of hydrogen and compounds of hydrogen 
and carbon. The quantity of these bodies thus generated 
is, however, in general so extremely small as to require the 
most sensitive reagents for their detection. 
At low temperatures.—Schonbein was the first to observe 
that nitric acid may be formed at moderately elevated or 
even ordinary temperatures. He obtained several grams 
of nitrate of potash by adding carbonate of potash to the 
liquid resulting from the slow oxidation of phosphorus in 
the preparation of ozone. 
More recently he believed to have discovered that ni- 
trogen compounds are formed by the simple evaporation 
of water. He heated a vessel (which was indifferently of 
* Meissner rejected Price’s test in the belief that it cannot serve to distinguish 
nitrous acid from peroxide of hydrogen, Hz Og. He therefore made the liquid 
to be exareined alkaline with a slight excess of potash, concentrated to small 
bulk and tested with dilute sulphuric acid and protosulphate of iron. (Unters. 
ii. a. Sauerstoff, p. 233). Schénbein had found that iodide of potassium is decom- 
posed after a little time by concentrated solutions of peroxide of hydrogen, but is 
unaffected by this body when dilute, (Jour. fir prakt. Chem., \xxxvi, p. 90). 
Zabelin agrees with Schénbein that Price’s test is decisive between peroxide of 
hydrogen and nitrous acid. (Ann. Chem. u. Ph., CXXX, p. 58.) 
