34^ BACTERIA. 



occurring in the water of the Tweed, contain a colouring 

 matter which has been described as resembling in a most 

 remarkable manner the aniline dyes ; in fact, in the case of 

 the pigment of the magenta micrococcus, the resemblance is 

 carried so far that in old cultures even the peculiar metallic 

 lustre of the aniline dyes is reproduced. Other pigments 

 are soluble in water, but not in alcohol, whilst the bacterio- 

 purpurin formed by Beggiatoa Roseopersicina is like chloro- 

 phyll, insoluble in both alcohol and water. The name 

 bacterio-purpurin, however, has been given by Engelmann 

 to the pigment produced or possessed by a whole group of 

 organisms. He concludes that it has, in these lower organisms, 

 the function of the chlorophyll of the higher plants. If 

 exposed on the microscope stage to the light of a sub-stage 

 spectroscope apparatus, these bacterio-purpurin bacteria 

 invariably tend to collect on that part of the slide that is 

 over the ultra-red bands of the spectrum, the portion of the 

 spectrum where the absorption of light by the bacterio- 

 purpurin occurred. He found that the analogy with plants 

 and chlorophyll became still closer from the fact that 

 wherever this occurred, oxygen was set free, and that 

 light in fact was necessary for- the continued existence 

 of these bacteria and for the development of their 

 characteristic colour - producing power. In the case of 

 this organism, according to Ray Lankester, the colour is 

 actually contained within the protoplasm of the organism, 

 where it appears in some cases to be combined with sulphur, 

 to form dark granules. In some cases the colour, inplace of 

 remaining within the organism, becomes diffused into the 

 surrounding media. As an example of this may be cited 

 the bacillus of " blue milk," which, growing along the track 

 of the needle in a gelatine culture, sends out a peculiar 

 iridescent green coloration into the surrounding gelatine ; 

 as time goes on this green is replaced by a smoky_ brown 

 colour. Similarly the Bacillus fiuorescens putidus_ imparts 

 to the gelatine, or any other material on which it is grow- 

 ing, a peculiar fluorescent green, and at the same time gives 

 off an odour of herring brine. 



That the decomposition of the sulphates in the presence of iron and 

 organic matter plays a most important part in the production of these pig- 

 ments has now been fully recognized. If it be borne in mind that sulphides 

 of the various metals appear as beautiful precipitates when thrown down from 



