148 PURPLE BACTERIA, 



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 sub- 

 stances 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 /A), 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 (A = 0.8 to 0.9 ju) 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 bac- 

 teria 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, notwithstanding 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 section. The in- 

 vestigation of the physiology of this central substance has yielded 

 a second series of weighty results, which will be given in Chapter 

 xxxv. 



