IRRITABILITY : INFLUENCE OF LIGHT 203 



cannot be supplied by growing the purple bacteria along with 

 these organisms. 



Both Molisch and Winogradsky have concluded that the 

 oxygen in Engelmann's experiments was derived from green 

 organisms accidentally present in the medium. It is evident 

 that the whole subject must be reinvestigated, before Engel- 

 mann's observations can be accepted. In his favour it may 

 be mentioned that Molisch's results were all negative, whilst 

 those of Engelmann were positive. In one experiment at 

 least the effect of contamination with green alga; can have no 

 part ; in the third experiment mentioned the bacteria are 

 shown clustered round a single individual of the purple 

 bacteria, and here there can be no question of the interference 

 of green organisms. 



Relationship of Light to the Growth of the Purple 



Bacteria. 



Engelmann sought to establish a relationship between 

 growth and light from the following experiment. When 

 tubes containing sea-wrack infected with purple bacteria are 

 left in the dark they remain colourless. When other tubes 

 of a similar kind are exposed to light the colour deepens. 

 When now the first set is exposed to the light the colour 

 develops after only a few days, and if the second set is now 

 placed in the dark the colour disappears. It is possible to 

 put another interpretation upon these results, for the mass of 

 individuals of the purple bacteria may remain stationary in 

 point of numbers, and develop or lose colour according to 

 whether they are placed in the light or the dark. Thus proof 

 is supplied of the dependence of colour on light but not neces- 

 sarily of the measurement of growth by colour. However, 

 Winogradsky (i), Molisch (3), and Skene have shown con-- 

 clusively on other grounds that the growth of these organisms 

 is dependent on light. The dependence is established by the 

 fact that the spread of the purple colour to cover fresh material 

 readily takes place in the light but not in the dark. Skene 

 cultivated purple bacteria in different coloured lights and 



