ASSIMILATION AND OXYGEN ELIMINATION. 147 



purpurin, showing a sharply defined band in the ultra-red (wave 

 length A = 0.9 to 0.8 /A) ; a second, less powerful, small band in 

 the orange (A = 0.61 to 0.58 /A) ; and, finally, a pale washed-out 

 band in the green (A = 0.55 to 0.52 /x). Engelmann then deter- 

 mined by an accurate quantitative photometric examination, with 

 the aid of the bolometric method, that a remarkable ratio prevails 

 between the intensity of the physiological action and the extent 

 of the absorption, i.e. the attractive force of a given colour of 

 the spectrum is greater in proportion as the latter is retained by 

 the colouring matter. From this is deducible the further con- 

 clusion, that the purple bacteria have great need, not merely of 

 light in general, but of certain components thereof in particular, 

 and especially those corresponding to the lines A , D, E, of the 

 spectrum. 



93. Assimilation and Oxygen Elimination. 



This behaviour of the purple bacteria, unique in the bacterial 

 kingdom, reminds one so much of the connection between light 

 and the activity of chlorophyll in the higher plants, that not only 

 does the question naturally arise as to the nature of the reactions 

 occurring under the influence of the selected rays, but also the 

 idea that here likewise there is assimilation accompanied by the 

 elimination of oxygen. This is, in fact, the case, the purple bac- 

 teria excreting oxygen in the presence of light. 



ENGELMANN (VI.) proved this in a variety of ways. One of 

 his experiments, which demonstrates it in a very elegant manner, 

 is based upon that already given in 41. Quiescent forms, re- 

 sembling a zoogloea, of one or other of these species are employed, 

 a portion about 2 sq. m.m. in size being placed in a drop of water. 

 To this are added a number of aerobic organisms (e.g. Spirillum 

 tenue, Sp. undula, various infusoria, &c.), capable of reacting on 

 even small quantities of oxygen, and the cover-glass is surrounded 

 with vaseline to prevent the admission of air. The oxygen dis- 

 solved in the water is very quickly exhausted, and the organisms 

 come to a standstill. If, now, the preparation be illuminated, it 

 will be seen directly that the wanderers scattered about rise and 

 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 phenomenon 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 Schizomycetes 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 



