H 4 PUHPLK BACTERIA 



hasten to the red skin where the oxygen they need is produced. On the other 

 hand, when replaced in the dark, they disperse again in all directions. That 

 oxygen is the attraction is demonstrated by the circumstance that the pheno- 

 menon just described does not occur when the experiment is performed on a 

 preparation the cover-glass of which is not made air-tight, and to which air can 

 consequently penetrate by diffusion. 



This peculiarity gives the purple bacteria a unique position amongst the 

 Schizomycetea as a connecting link between them and the green plants. From 

 its capacity of converting the actual energy of light into potential chemical 

 energy, and of changing vibrations of light into force, bacterio-purpurin deserves 

 the title of a true chromophyll, since it plays in the purple bacteria the same 

 role as chlorophyll does in green plants. These two substances are antithetical, 

 accomplishing similar tasks by different methods of working. On more closely 

 examining the individual spectral colours for their power of eliminating oxygen, 

 this latter faculty is found to be proportional to the absorptive capacity of 

 bacterio-purpurin for the colour in question. The maximum effect is produced 

 in the case of the aforesaid ultra-red rays (X = 0.8-0.9 /*) whilst, on the other 

 hand, the rays between the lines B and C are inert. With chlorophyll the 

 converse is the case, this being quite inactive in the ultra-red rays, and exerting 

 its greatest effect in the red rays (between B and C). 



The aforesaid ultra-red rays (\ = 0.8 to 0.9 p) are well known under the 

 name of " invisible heat rays," being inappreciable to the eye as light. The 

 discovery that they are the rays that not only enable the purple bacteria to 

 exhibit activity, but also spur the latter on to their highest degree of efficiency, 

 allows the wider conclusion to be drawn that the elimination of oxygen through 

 the activity of the vegetable cell is not dependent on the co-operation of visible 

 light rays, but may also proceed in the dark. 



So far for the facts ; but looking beyond them, it may be asked if the faculty 

 of absorption in the dark is inseparably connected with the presence of bacterio- 

 purpurin, or if there are also colourless bacteria similarly endowed. The answer 

 to this query will be found in chapter xxxvi., which treats of nitro-bacteria. 

 This is a matter of such great interest as to deserve special consideration ; at 

 present, therefore, we will merely review the facts hitherto discovered in the 

 case of the purple bacteria, their general importance being so great that we 

 shall certainly not regret having bestowed attention on these organisms, not- 

 withstanding that they are devoid of technical application. The scientific 

 harvest they are capable of yielding is, however, still far from being exhausted. 

 It will be remembered that it is in the outer layer of the inner substance of the 

 cell of these red organisms that this very influential bacterio-purpurin has its 

 abode, and that the central substance is surrounded by this layer. The study 

 of this from a morphological standpoint by Biitschli led to the conclusions 

 respecting the structure of bacterial plasma already recorded in an earlier sec- 

 tion. The investigation of the physiology of this central substance has yielded 

 a second series of weighty results, which will be given in chapter xxxv. 



