52 NITROGEN. DECOMPOSITION OF NITROGENOUS COMPOUNDS. 
matter, which, having become lifeless, can serve them as food. 
Every bit of excreted urea, even that secreted by the smallest insects, 
every dead animal body, every bit of vegetable matter whether it 
be leaf, branch, or fruit, provided it contain proper moisture, is sure 
to be appropriated as food by some of these ubiquitous putrefactive 
bacteria. The material is used as food by the microorganisms, and, 
as a consequence, they multiply rapidly within the decaying sub- 
stances, developing vigorously for a time. After they have used 
up the food, their growth is checked and some of them remain 
ready to grow again when more organic matter comes within their 
FIG. 14. Proteus vul- FIG. 15. Com- 
garis, a common bacter- mon decomposition 
ium of decomposition. bacteria. B. fluor- 
escens and B. sub- 
tilis. 
reach. By their action, then, every bit of organic matter which 
reaches the soil is seized and rapidly decomposed. 
The chemical nature of these destructive changes is very 
complicated and highly varied. It will be a long time before our 
chemists understand them, for they involve problems irr physiolog- 
ical and organic chemistry yet unsolved. We know that many new 
products are formed, and that these new products must be regarded 
as belonging to at least two types, so far as concerns their relation 
to the bacteria. Some of them must be regarded as secretions or 
excretions from the bacteria and hence as the result of the active 
metabolism of the microorganisms. These are probably rather 
small in amount, but of great significance in some connections, 
inasmuch as many of them are poisonous. Others must be looked 
upon as by-products of decomposition. By this is meant that, as 
the bacteria take certain atoms from the complex molecules for 
their own use, the rest of the molecule can no longer retain its 
