78 BACTERIOLOGY. - 
chemical decomposition of the butter by the oxygen of 
the air under the influence of sunlight and (2) through 
fermentation by the lactic acid of the milk-sugar left 
in the butter. Fats are, however, attacked by bacteria 
when mixed with gelatin and used as culture media, 
with the consequent production of acid. 
Putrefaction. By putrefaction is understood in com- 
mon parlance every kind of decomposition due to bac- 
teria which results in the production of malodorous 
substances. Scientifically considered, putrefaction de- 
pends upon the decomposition of complex organic 
compounds, albuminous substances, and the like (glue, 
albuminoid bodies), which are frequently first pepton- 
ized and then further decomposed. Typical putrefac- 
tion occurs only when oxygen is absent or scanty; the 
free passage of air through a culture of putrefactive 
bacteria—an event which does not take place in natural 
putrefaction—very much modifies the process: first, 
biologically, as the anaérobic bacteria are inhibited, 
and then by the action of the oxygen on the products 
or by-products of the aérobic and facultative anaérobic 
bacteria. 
As putrefactive products we have peptone, ammonia 
and amines, leucin, tyrosin, and other amido substances. 
Oxyfatty acids, indol, skatol, phenol, and, finally, sul- 
phuretted hydrogen, mercaptan, carbonic acid, hydro- 
gen, and, possibly, marsh-gas (H,C). 
According to recent observations, nitrification is 
_ produced by a small, special group of bacteria, culti- 
vated with difficulty, which do not grow on our usual 
culture media. From the investigations of Winograd- 
sky it would appear that there are two common micro- 
organisms present in the soil, one of which converts 
