150 



OUTLINES OF BACTEEIOLOGY 



to obtain the energy it needs for carrying on all the processes of 

 life, so that the sulphur-bacteria obtain the same benefit from this 

 process that other bacteria obtain by respiration. It is obviously not 

 identical with respiration, for it does not involve the breaking down 

 of protoplasm. As the end product is sulphuric acid, growth would 

 soon be inhibited altogether if this substance were allowed to accumu- 

 late. The acid, however, is neutralised by the acid carbonate of lime, 

 CaH2(C03)2, which is usually present in the kind of water in which 

 these organisms grow. A very interesting result follows from the fact 

 that the two gases which are necessary for the existence of these 

 bacteria, viz. sulphuretted hydrogen and oxygen, are found, the 

 former at the bottom where decomposition is taking place, the latter 



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at the top where the liquid is in contact with the atmosphere. As 

 both are necessary the organisms are found at a level in the liquid 

 where oxygen has penetrated downwards and sulphuretted hydrogen 

 upwards. If the latter gas is profusely developed the organisms can 

 even be found at the surface, while on the other hand if the supply 

 be very small, they are found at the bottom of the vessel in which 

 they are growing. Some beautiful experiments have been performed 

 by Winogradsky and Jegunow. By varying the supply of these 

 gases, they have been able to control the growth-level taken up by the 

 organisms. We may note here the life-history of the sulphur molecule. 

 As is well known, matter cannot be destroyed. The sulphur is bound 

 in a complicated fashion in the albuminoid molecule in the animal 

 or vegetable body. When the organism dies, complicated changes 

 take place through the agencies of bacteria, resulting in the liberation 



