PHYSIOLOGY OF THE SULPHUR BACTERIA 285 



narrow tongue of land. Their bottom is covered by a thick mud, which owes its 

 black colour to the FeS thrown down from the iron compounds in the water 

 (and in the plants rotting therein) by the sulphuretted hydrogen generated from 

 sulphates by the reducing action of bacteria investigated byE. 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 ; 

 by 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 2 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 generated. This 

 change of feeding-ground cannot, however, be followed by all species of sulphur 

 bacteria, since just in the same way 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 fluctuations of atmospheric pressure 

 will suffice to produce a change in the predominating species of a diversified 

 mixture of sulphur bacteria 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 re- 

 searches of M. JEGUNOW (I.) on the colourless non-filamentous 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 contact 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 artifically induced the processes going on 

 in the Limanes to repeat themselves so far as necessary to the purpose in view 

 on a small scale in the laboratory, by placing a certain quantity of the black 

 mud in suitable vessels containing water, and then leaving the whole to stand 

 uncovered. We will not go further into the matter of the rise and fall of the 

 bacterial plane as observed by him, because BEYERINCK (I.) had made similar 

 experiments two years earlier, and applied to the phenomenon a term (Bakterien- 

 Niveau) having the same significance as that used by Jegunow. 



The discoveries made by JEGUNOW (II.) with regard to the construction of 

 this bacterial plane, in the case of the organisms now in question, must, however, 

 be considered as novel. When cultivated in higher and broader, but thinner, 

 strata of liquid, the plane assumes the form reproduced on a reduced scale in 

 Fig. 79, i.e. the bacteria do not form a simple plane, but become piled up in 

 places into tuft-like projections each about 3-4 mm. long four of these being 



