SULPHUR BACTERIA 67 



are not able to live in water containing SH 2 ; they perish where Chromatium 

 or Beggiatoa thrives. 



Seeing that a solution of SH 2 in water is very easily decomposed by the 

 oxygen of the air* and free sulphur deposited, the sulphur bacteria would 

 be able to take advantage of this oxidation for their life-processes, even if 

 they merely had the power of existing in H 2 S solution. The H 2 S entering 

 their cells would be oxidised by absorbed atmospheric oxygen, and the 

 sulphur set free would represent an abundant source of energy for further 

 oxidation. To explain the facts observed we need only assume further that 

 the protoplasm increases the oxidizing power of the atmospheric oxygen 

 and renders it ' active.' The amount of energy gained is very consider- 

 able. The oxidation of the dissolved SH 2 gives seventy-one calories, and 

 the further conversion of the freed sulphur into sulphuric acid no less than 

 2,109 calories. 



The fact that the thiobacteria can dispense entirely with organic com- 

 pounds which might be burnt up to CO 2 , together with their inability to 

 live without sulphur, shows that it is the oxidation of the sulphur alone 

 which takes the place of the respiratory processes of other organisms. 



The two great physiological processes, the assimilation and storage 

 of respirable material on the one hand, and the liberation of energy by 

 its oxidation (respiration) on the other, may be represented in a tabular 

 form. The following scheme shows the processes in green-plants, meta- 

 trophic bacteria and sulphur bacteria : 



ASSIMILATION OF RESPIRABLE MATERIAL. 



* In presence of cotton-wool or some other porous body it is even oxidized to H 2 SO 4 . 



