COLOURING MATTER OF SULPHUR BACTERIA 237 



possible to fix the same middle point for the bands both 

 of weak and of strong solutions of the same fluid. This 

 is, however, not the case, for the absorption bands change 

 their position even with changes in the specific gravity of 

 the solvent, as was pointed out by Kraus. 

 (iii) The number and intensity of the bands vary with the 

 solvent. 



The Colouring Matter of Thioporphyra volutans. 



The cultures vary from a pinkish-purple to a deep violet- 

 purple. The same culture frequently shows a variation of colour 

 during its periods of active multiplication. Like other coloured 

 sulphur bacteria it is aftected by the intensity of the light to 

 which it is exposed. Bright light is deleterious to its pigment 

 as it is to chlorophyll. On the other hand, during periods when 

 the intensity of daylight is low, as in the winter season in north 

 temperate climates, the colour is not developed. It begins to 

 form in the early summer and persists until late autumn. 

 This seasonal appearance is doubtless correlated with such 

 factors as temperature, presence of dissolved organic matter, 

 etc., which determine the growth of the organism. The 

 absence of colour is not due to the absence of colouring matter, 

 but to the absence of the organism in an active state of multi- 

 plication. 



Spectroscopic Examination of the Colouring Matter 

 OF Thioporphyra volutans. 



By the use of different solvents two coloured substances 

 were separated :— 



1. A purple colouring matter, soluble in chloroform and 

 in carbon bisulphide. 



2. A brown or greenish-brown colouring matter, insoluble 

 in chloroform and in carbon bisulphide, but soluble in alcohol 

 and readily soluble in petroleum ether. 



The purple colouring matter is unstable, and oxidizes in 

 a few clays to a brownish-purple colour. 



When the alcohol extract is shaken with petroleum ether 

 a yellowish substance separates from the alcohol, and tinges 



