MICROBES, MOULDS, AND YEAST. 203 



more seeds, if bacteria had been introduced. Another 

 observer gives the actual amount of nitrogen gained by 

 the bacteria. After twenty days' growth in a special 

 solution (500 ccm. of liquid), one form, Clostridium, had 

 been able to obtain 28.87 milligrams of free nitrogen. 

 The nitrate deposits found in South America are, 

 probably, due to Bacteria. 



These nitrogen-gathering bacteria (B. 5, p. 128) 

 belong to several species and no doubt there is often a 

 sort of co-operation amongst them. Sometimes, they 

 may be assisted by other plants, such as Algae 

 and Mosses, but this has not been clearly proved. In 

 other cases, the bacteria act in a reverse way, as 

 one would expect (see diagram on p. 128); for the 

 atmosphere could not remain unchanged, if nitrogen were 

 always being taken from it and none returned. These 

 other soil bacteria, B. 4, change nitrates into free Nitro- 

 gen. They are specially abundant where oxygen is kept 

 out, as, for instance, in manure heaps which have been 

 kept too long. Races of bacteria soon develop in such 

 heaps, and work upon the decaying material, turning the 

 nitrogen compounds into free nitrogen. As manure is 

 often stored for six months, this means a serious loss to 

 the farmer. It has been calculated that, in Germany 

 alone, ;^ 15,000,000 worth of nitrates is lost every year 

 by delay in using the manure. 



When waste products are spread upon the fields, they 

 are attacked by quite a different series of forms ; some 

 are very beneficial, as they break up the nitrogenous 

 bodies first into nitrites, B. 2, and then into nitrates, 

 B. I. These nitrates enter the plant, and, if the plant 

 is afterwards eaten by an animal, these nitrogenous 

 substances are utilised in the animal's body. All 

 decaying vegetable and animal substances, such as 



