MICROBES, OR BACTERIA. 121 
XI. THe MicroBes WHICH PRODUCE SALTPETRE. 
It is known that nitre or saltpetre, 1.e. potas- 
sium nitrate, is produced in damp places where de- 
composing animal matter is found in contact with 
carbonate of potassium. It is found, combined with 
other salts of lime, soda and magnesia, in stables, sheep- 
folds, cellars, in the neighbourhood of urinals, and 
in the earth of some localities (Peru and Chili), Its 
industrial importance in the manufacture of gun- 
powder, etc., has led to its collection. Formerly it 
was extracted from the plaster of old houses, or from 
artificial nitre works which combined conditions 
favourable to its production. Nitrates are produced 
by the gradual oxidation of the ammonia furnished 
by animal excretions. For along while it was supposed 
that this oxidation was simply due to the influence 
of porous bodies, such as earth and stone walls. Nitric 
acid was produced, then nitrates of lime, potas- 
sium, ete. 
The researches of Boussingault, Schloesing, and 
others, have now taught us that this phenomenon of 
organic chemistry is due, like many others, to the 
vital activity of one or more species of microbe, 
whose invariable presence in the natural or artificial 
nitre-works has been ascertained. These microbes are 
aérobies, 7.e. they only live and work when in contact 
with the oxygen of the air, from which they derive 
