140 Aquatic Organisms 



city's waste. But there is no natural water free from 

 them. Let a dead fly fall upon the surface of a tumbler 

 of pond water and remain there for a day or two and it 

 becomes white with water mold, whose spores were 

 present in the water. Let any organic solution stand 

 exposed and quickly the evidence of rapid decomposition 

 appears in it. Even the dilute solutions contained in a 

 laboratory aquarium, holding no organic material other 

 than a few dead leaves will often times acquire a faint 

 purple or roseate hue as chromogenic bacteria multiply 

 in them. 



Bacteria — ^A handful of hay in water will in a few 

 hours make an infusion, on the surface of which a film 

 of "bacterial jelly" will gather. If a bit of this ''jelly" 

 be mounted for the microscope, the bacteria that secrete 

 it may be found immersed in it, and other bacteria 

 will be found adherent to it. All the common form- 

 types, bacillus, coccus and spirillum are likely to be seen 

 readily. Thus easy is it to encourage a rich growth of 

 water bacteria. Among the bacteria of the water are 

 numerous species that remain there constantly (often 

 called "natural water bacteria"), commingled at certain 

 times and places with other bacteria washed in from the 

 surface of the soil, or poured in with sewage. From the 

 last named source come the species injurious to human 

 health. These survive in the open water for but a short 

 time. The natural water bacteria are mainly beneficial ; 

 they assist in keeping the world's food supply in circula- 

 tion. Certain of them begin the work of altering the 

 complex organic substances. They attack the proteins 

 and produce from them ammonia and various ammonia- 

 cal compounds. Then other bacteria, the so-called 

 "nitrifying" bacteria attack the ammonia, changing it 

 to simpler compounds. Two kinds of bacteria succes- 

 sively participate in this: one kind oxidizes the 



