126 LIVING ORGANISMS OF THE SOIL [chap. 



putrefactive breakdown another set of bacteria takes up 

 the resulting nitrogenous compounds and breaks them 

 down still further into ammonia, whereupon the nitrifying 

 bacteria can begin to exercise their function. Com- 

 pounds in the soil are, however, subject to other kinds 

 of change, as we can show by a further experiment. A 

 flask of about lOO c.c. capacity is fitted with a good cork 

 and a single exit-tube leading downwards and then 

 bent up at the extremity: into this flask about lO 

 grammes of soil and a \ gramme of sugar are placed, 

 then the flask is completely filled with a solution 

 containing i per thousand of potassium nitrate. After 

 the contents of the flask have been shaken a little to 

 get the sugar dissolved, the cork and exit-tube are 

 inserted ; the latter is completely filled with water and 

 set up so that it dips into water and is directed into the 

 open end of an inverted test-tube, also filled with water. 

 The apparatus is then put away in a dark, warm place, 

 and examined from time to time ; after some days 

 bubbles of gas will begin to appear and will collect in 

 the test-tube. When enough of the gas has been 

 collected it may be examined with a lighted taper, and 

 by shaking up with lime water it will prove to consist 

 in the main of nitrogen gas. If at the same time the 

 liquid remaining in the flask be tested it will no longer 

 show any reaction for nitrate; this salt has been 

 decomposed by certain organisms in the soil with the 

 liberation of nitrogen gas. Obviously this is a very 

 wasteful process, because nitrates and all forms of 

 combined nitrogen are valuable, whereas four-fifths of 

 the atmosphere is made up of nitrogen gas. Any such 

 change in cultivated soils ought so be checked as much 

 as possible, and this can only be done by avoiding the 

 conditions which give rise to the decomposition. Now 

 the essential conditions of the experiment were the 



