206 PLANT LIFE. 



indigo, is obtained from the leaves and young stems 

 of the indigo by a sort of fermentation process, almost 

 certainly due to bacteria. (3) The characteristic 

 qualities of tobacco are only developed in the leaf 

 by the action of bacteria, whose exact function 

 is not known. (4) The manufacture of china 

 involves getting the help of bacteria, as the change 

 from kaolin X.o porcelain is assisted by them. (5) The 

 necessity of creosoting, or tarring, timber, especially if 

 it is to be placed in the soil, only arises because 

 of the bacteria and other fungi, which would otherwise 

 soon attack and destroy the dead wood. (6) Wine^ 

 Beer^ etc., are kept corked and have to be very care- 

 fully manufactured on account of the ever-present 

 danger of bacterial infection. (7) Bacteria have also 

 been used by man for the purification of sewage. 

 They swarm in this material. London sewage contains 

 at least 49 distinct species, and they are present in all 

 sewage in enormous numbers. 



A short summary may be given of the system of puri- 

 fication adopted. The liquid is passed into a large tank 

 or series of tanks and allowed to settle. During this 

 stage of purification, bacteria, which are unable to exist in 

 oxygen, act upon the material forming nitrites and 

 removing about 5 3 per cent, of the impurities. The liquid 

 then leaves this settling tank and is passed through an 

 accumulation of coarse large-sized coke fragments or 

 •' cokebreeze!' These pieces are covered by a film of 

 bacterial colonies, and the bacteria manufacture nitrates 

 from the passing sewage and so remove a further 29 per 

 cent, of the impurities. It is not possible to keep the 

 colonies continually at work, as they must be allowed to 

 rest, and to recover from their labours. Hence a double 

 set or shift of these last coke beds is required. It will 



