122 LIVING ORGANISMS OF THE SOIL [chap. 



the change was understood men had learnt that it was 

 forwarded by a proper degree of moisture, by warmth, 

 and by the presence of a base like carbonate of lime. 

 Not, however, until about 1 877 was it demonstrated that 

 the agents effecting the change are certain minute 

 organisms living in the soil and called bacteria, which are 

 classed with the plants, though they are so lowly in type 

 that they might almost be regarded as the common 

 meeting-ground of the lines of plant and animal descent. 

 At first only the fact was demonstrated that the change 

 was due to something living, and this was done by 

 showing that nitrification ceases when the soil is kept 

 permeated by the vapour of chloroform, which will 

 inhibit or destroy living organisms. Again, if the soil is 

 heated to the temperature of boiling water it loses its 

 power of forming nitrates. The first step having thus 

 been taken, subsequent investigators cleared up the 

 various steps in the process and succeeded in isolating 

 from the soil in an unmixed state two organisms, one of 

 which will change ammonia into nitrite and the other 

 will complete the oxidation into nitrate. We can 

 demonstrate their action by a few simple experiments ; 

 an ammoniacal solution that also contains the other 

 elements necessary to the nutrition of bacteria is made 

 up as follows : — 



2 grammes Ammonium Sulphate 

 0-2 „ Pot.assium Phosphate 

 o-i „ Magnesium Sulphate 

 O'l „ Sodium Chloride 



are dissolved in i litre of water, and a few drops of 

 ferric chloride solution are also added. One hundred 

 cubic centimetres of this solution are placed in each of 

 four flasks of about 300 c.c. capacity, the mouths of 

 which are afterwards plugged with cotton-wool. The 

 flasks and their contents are then sterilised by heating 



