128 LIVING ORGANISMS OF THE SOIL [chap. 



changes brought about by bacteria in the nitrogen- 

 ous compounds present in or added to the soil, but 

 non-nitrogenous materials undergo equally marked 

 decompositions, often of great practical importance. To 

 illustrate the nature of these changes, two experiments 

 may be performed. A tall bottle, which has an inlet at 

 the bottom, is filled with dry soil, mixed with a small 

 quantity of finely powdered sugar, about 2 grammes of 

 sugar per kilogram of soil. The action is the same 

 with all carbohydrates, but it will take place more 

 rapidly with sugar, so that it is the preferable substance 

 to use. Sufficient water is slowly poured in to moisten 

 the soil, then the bottle is put aside in a warm place, 

 and once a day the gas it contains is pumped out 

 through a wash-bottle containing lime water, air being 

 allowed to enter in its place. It will be found that 

 quantities of carbon dioxide are given off by the soil, 

 and in a week or two if the soil be washed and the 

 solution filtered and tested no trace of sugar will be 

 detected. It has all been oxidised by the bacteria of 

 the soil into carbon dioxide, air of course having been 

 also necessary to the process. In the other experiment 

 a corked-up flask with an exit-tube, such as was 

 employed in the denitrification experiment, is required. 

 The flask is partly filled with a solution of nutrient salts, 

 such as was used for water cultures, and a number of 

 strips of clean filter paper, which is nearly pure cellulose, 

 is introduced together with a small quantity of soil, or 

 better still, of pond mud. The flask and tube are filled up 

 with water, and made to lead into an inverted test-tube 

 over water as before, then the whole apparatus is put 

 aside in a warm place in the dark. It may be some 

 days before any action sets in, but eventually bubbles 

 of gas will arise and will collect in the test-tube, which 

 can be removed for their examination from time to 



