THE CIRCULATION OF NITROGEN IN NATURE. 59 
If an electric spark be passed through a vessel containing 
air, red fumes are produced which readily dissolve in water 
with formation of nitrous and nitric acids. This latter is 
identical with that obtained from saltpetre or nitre by heat- 
ing it with sulphuric acid. Hence the name nitrogen, or 
nitre generator. Nitre then is the salt produced by neutral- 
izing nitric acid with the alkali potash, and is strictly 
potassium nitrate ; but if we employ the name generically 
we can understand by it any neutral salt formed by acting 
upon nitric acid with a base such as lime or magnesia, 
and we have lime nitre or calcium nitrate, magnesium nitre, 
silver nitre, and so on. 
Now if we withdraw a portion of the oxygen from a nitre, 
which can easily be done by heating either alone or with a 
substance which will abstract oxygen, we form a second 
series of salts known as nitrites, of which potassium nitrite 
is the type, and of which we can produce a complete series 
corresponding to the nitrates. 
If, however, we carry this reduction so far as to remove 
all the oxygen, we liberate the nitrogen as the elementary 
gas. 
But if we at one and the same time strongly reduce, and 
present a sufficiency of hydrogen, we can combine the 
nitrogen with the hydrogen and form ammonia. 
Further, ammonia can be produced by the direct union of 
nitrogen and hydrogen under the influence of the silent dis- 
charge ; the ammonia can be directly oxidized by platinum 
black into nitrous acid, and the latter further oxidized into 
nitric acid by permanganate of potash, and so we can com- 
plete the inverse series of changes. In this way we can 
ring the changes on these substances, and, given one of them, 
can produce all the others, but only by processes which are 
not in force to any appreciable extent in nature. 
