VII.] SOIL BACTERIA 125 



smell will have disappeared and been replaced by a 

 faint smell of ammonia, the next step in the cycle of 

 transformations. Another month in the dark will show 

 the third and, as a rule, the final stage well on its way — 

 the previously formed ammonia is now being converted 

 into nitrate. Lastly, the fate of the nitrate may be illus- 

 trated by leaving the flask in the light for a few weeks, 

 whereupon a bright green growth of algae will appear 

 from spores introduced with the original soil. In 

 sunlight the algae will form a scum buoyed up by the 

 bubbles of oxygen set free, because the algae are green 

 plants which possess chlorophyll and split up carbon 

 dioxide like other higher plants. After the algal growth 

 has made some headway the liquid may be tested, and 

 will no longer show either nitrate or ammonia ; these 

 substances have been absorbed by the growing algae 

 and reconverted into proteins by being drawn into 

 combination with the carbon compounds which the 

 algae have assimilated from the atmospheric carbon 

 dioxide. Thus in the flask has been illustrated the 

 whole cycle of the breakdown by bacteria and the 

 reconstruction in the living plant of the protein bodies 

 which are fundamental to both plants and animals. 

 This all-important set of changes is brought about by 

 at least three distinct sets of bacteria. The first com- 

 prises numerous forms or species of the so-called 

 putrefactive bacteria, which seize upon the complex 

 nitrogenous bodies like the proteins and break them 

 down into simpler substances, among which are certain 

 compounds containing sulphur and possessing character- 

 istic disagreeable smells, the unpleasantness of the smell 

 being probably nothing more than a physiological 

 memory of the fact that intestinal disorders have always 

 followed the consumption of food possessing such a smell 

 and therefore swarming with bacteria. After this first 



