CHAPTER XIII. 



THE COLOURING MATTER OF THE SULPHUR 

 BACTERIA. 



Introduction. 



All the chromoparous sulphur bacteria arc purple, and 

 range from the pink-purple colour of Chromatium to the plum- 

 purple colour of well-developed cultures of Thioporphyra 

 voliitans. The same culture does not invariably retain the 

 same shade of colour, but frequently passes from one extreme 

 of purple to the other. These shades are due to varying 

 proportions of a purple pigment, named hacteriopurpurin, 

 and a yellow-green pigment, bacteriochlorin. Again, these 

 two pigments are compound, not simple substances. Their 

 chemical composition has not yet been determined. 



Historical. 



Ray Lankester (i — 2) was the first to investigate the colour- 

 ing matter of the purple bacteria, which he named bacteriopiir- 

 piirin. This substance was stated to be insoluble in water, 

 alcohol, ammonia, acetic acid, and sulphuric acid. In one 

 place the pigment is said to be insoluble in chloroform, and in 

 another that this reagent changes it to an orange-brown colour, 

 which gradually dissolves. In point of fact the pigment is 

 soluble in chloroform in the dry state, insoluble in the wet 

 state. It is also soluble in alcohol. 



Ray Lankester records the following results of his 

 spectroscopic analysis : — 



1. End absorption in violet. 



2. Two absorption bands, one near the D line, and the other 

 near the E line (see Fig. 65, I). 



Interest in the subject was quickened by Engelmann's 



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