372 THE SULPHUR BACTERIA. 



plants rotting therein) by the sulphuretted hydrogen generated 

 from sulphates by the reducing action of bacteria investigated by 

 E. BRUSILOWSKY (I.). The red sulphur bacteria are but rarely 

 found in mineral sulphur springs. According to Cohn, they have 

 "been detected by Morren in the sulphur spring at Ougree, on the 

 Maas ; by Fontane and Jaly, in that at Sales, in the Pyrenees ; 

 fay Meneghini, in that of the Euganean Hills, near Padua ; and 

 by Cohn himself in that of Tivoli, near Rome. 



The existence of the sulphur bacteria is often a very hard one, 

 because it requires the simultaneous presence and availability of 

 two gases which neutralise one another and become converted into 

 sulphur and water 



H a S + O = H 2 + S. 



So that actually the surface of liquids wherein H 2 S is produced 

 in abundance by the activity of reducing bacteria becomes coated 

 with sulphur formed by purely chemical means, in accordance 

 with the foregoing equation. Now, in order that the sulphur 

 bacteria may be in a position to exert their powers of oxidation, 

 it becomes necessary for them to inhabit certain strata of the 

 liquid between the limits where the oxygen can gain access from 

 above and sulphuretted hydrogen reach them from below. If the 

 liberation of the latter gas goes on briskly, this level rises, and 

 may ascend to the surface of the liquid ; otherwise it sinks and 

 approaches the bottom, where the sulphuretted hydrogen is gene- 

 rated. This change of feeding-ground cannot, however, be followed 

 "by all species of sulphur bacteria, since just in the same wny as 

 has been explained with regard to sulphuretted hydrogen these 

 organisms are adapted to a certain tension of oxygen, which varies 

 in the different species, i.e. they cannot stand the presence of more 

 than a certain quantity per unit of volume of the liquid. In the 

 case of oxygen, this tension is naturally greatest at the surface and 

 smaller at greater depths. It will be evident that even the fluctua- 

 tions of atmospheric pressure will suffice to produce a change in 

 the predominating species of a diversified mixture of sulphur bac- 

 teria in their natural haunts. The same applies to the rate at 

 which the sulphuretted hydrogen is disengaged. 



For an instructive insight into these conditions we are indebted 

 to the researches of M. JEGUNOW (I.) on the colourless non-fila- 

 mentous species referred to at the close of the last paragraph. As 

 already stated, the habitat of the sulphur bacteria is in those 

 strata of the liquid where the oxygen from above comes into con- 

 tact with the sulphuretted hydrogen from below. At this level the 

 organisms congregate to form an assemblage visible to the naked 

 eye, and which the above-named Russian physiologist termed the 

 "bacterial plane, the structure of which he examined minutely. He 

 artificially induced the processes going on in the Limanes to repeat 

 themselves so far as necessary to the purpose in view on a 



