324 ECOLOGY 



isolate them from the soil, or induce them to grow on an 

 artificial food-substance, so that we may examine them ? 

 The following experiment enables us to do this : 



Sterilize some water by boiling, cover it to keep out dust, 

 and allow it to cool. Thoroughly clean and sterilize, bj' 

 boiling, seven test tubes and three small dishes (Petri dishes 

 are the most convenient), invert them to drain off the water, 

 and close each tube with a plug of clean cotton wool. 

 Label the tubes A to G and the dishes I, 2, and 3. Boil 

 a leaf of gelatine in a little water, and with this half-fill tubes 

 A, B, and C. Pour A into dish No. 1 and cover. Take a 

 little garden soil, sterilize part of it by baking, leaving the 

 rest untreated. Half-fill tubes D and E with sterilized 

 water and to D add a little baked soil and shake up. With 

 a sterilized pipette transfer a few drops of the muddy liquid 

 from tube D to E to dilute it. Pour several drops of this 

 into the gelatine in tube B, mix them, pour into dish No. 2, 

 and cover as before. 



Using the remaining tubes, repeat this with the ordinary 

 unbaked soil and pour the mixture of gelatine and diluted 

 soil-water into dish No. 3. Cover and set all three aside in 

 the dark for a day. Look at them occasionally and com- 

 pare the results. How do you account for the differences ? 



If the experiment has been carried out carefully, Nos. 1 

 and 2 remain unchanged, but in No. 3 a number of specks 

 appear on the gelatine, which soon increase in size, and form 

 distinct patches or colonies. If a little of one of these be 

 examined under the microscope, myriads of tiny rods or 

 bacteria will be seen moving in the liquid. These, together 

 with the moulds, are the living beings of the soil which 

 decompose the organic matter, and prepare an important 

 part of the mineral food which green plants take up in 

 solution. Culture No. 1 shows they were not present in the 

 boiled gelatine, and No. 2 shows that the bacteria in this 

 part of the soil were killed by baking. 



