COLOURING MATTER OF SULPHUR BACTERIA 239 



spite of these spectroscopic differences the colouring matter of 

 this microbe is not essentially different from that of the other 

 sulphur bacteria. Its similarity is shown by its behaviour to 

 the various solvents, and by the shape and colour of the 

 crystals formed from the purple extract. 



To the doubts that have been cast on the value of absorption 

 spectra as a mode of investigation, the following must be added. 

 When the same purple fluid, containing a preponderating 

 percentage of colouring matter from Thioporphyra, was daily 

 examined, the spectrum gradually changed, so much so, that 

 quite a different spectrum was presented at the end of a week. 

 This might be attributed to chemical changes during the period, 

 although such were not apparent, but when the same prepara- 

 tion was again made from the same material and using the 

 same solvents there was frequently no similarity in the ab- 

 sorption spectra. (Compare VI, VH, VIII, Fig. 65.) When 

 cultivated on cardboard the absorption spectrum shown in 

 Fig. 65, IX, was obtained. In this there is nothing to observe 

 except a general darkening on both sides, which is more 

 pronounced on the violet end. The spectra obtained by 

 cultivation on cardboard differed from those obtained by 

 cultivation on Fucus. These spectra, however, showed 

 that in the colouring matter of Thioporphyra vohitans both 

 chlorphyll and phycoerythrin (Fig. 65, X) are absent. 



Spectro-photometric Method of Examination. 



By the use of the apparatus described below it is possible 

 to measure the amount of light absorbed at any desired wave- 

 length. Two spectra, one above the other, are viewed at the 

 same time by an arrangement which may be understood by 

 reference to the accompanying diagram (Fig. 66). The light 

 in one passes through the solution, in the other it does not. 

 By means of a cap the whole light can be cut off except for that 

 which comes through a small vertical slit with which the cap 

 is fitted, and which exposes a small area of each spectrum. 

 As the cap can be made to travel over the whole length of the 

 spectrum a comparison can be made of the intensity of the 

 light from the two spectra at any selected point. 



The light from Li passes through a glass screen (AJ, the 



