374 THE SULPHUK BACTEKIA. 



tion of these tufts by the aid of a horizontal microscope shows 

 that they are formed by the movement of the individual bacteria, 

 in a manner similar to the gushing of a spring ; they descend in 

 the axis of the tuft, and then describe an arc in their return to the 

 plane. When inverted by the microscope this resemblance is still 

 more striking ; so that Jegunow has styled the planes " fountain 

 planes." The velocity of the individual cells he found to be 

 0.02 mm. per second. In tracing the chemical activity of the 

 bacteria, he made use of a simple and reliable reagent for sulphu- 

 retted hydrogen : a fine (woollen or similar) thread treated first with 

 ferric chloride and then with ammonia, both in such a very dilute 

 condition that the thread is stained merely a pale yellow. A glass 

 weight is then attached to the thread and let down into the liquid, 

 whereupon the lower part of the thread, as far as the summit of 

 the tufts on the fountain plane, quickly turns black, from the 

 formation of FeS. From that point onwards, however, the colour 

 gradually changes to white. This experiment shows that in the 

 summits of the tufts the sulphuretted hydrogen arising from 

 below is first oxidised to sulphur, and stored up in the cell, 

 which conveys it to a higher level (the actual plane), and there 

 oxidises it to sulphuric acid. This acid then dissolves the ferric 

 oxide on the upper part of the thread, which is consequently de- 

 colorised at this level. The time occupied by the cells in making 

 a single trip and therefore also the total period required for the 

 conversion of H 2 S into S0 3 and the expulsion of the latter from 

 the cell was ascertained by Jegunow to be about five minutes. 



The importance of the sulphur bacteria in the economy of 

 Nature is unmistakable : in co-operation with the sulphate-reducing 

 bacteria they ensure that the sulphur cycle pursues an uninter- 

 rupted course, the element being taken up by the higher plants in 

 the condition of sulphates, and deposited in the cells in the form 

 of organic compounds, from which, in the course of putrefaction, 

 it is liberated as sulphuretted hydrogen, and is finally then re- 

 converted into sulphates by the sulphur bacteria and recommences 

 its course through the higher plants. 



