ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 381 



As the Spirilla greatly predominated, as much in number as in 

 volume, the author had at first considered it probable that it was 

 they which disengaged oxygen under the influence of light. But 

 Spirilla, even in thick layers, are quite colourless. We should, there- 

 fore, have here the unheard-of fact of a disengagement of oxygen 

 without the agency of chlorophyll or of some pigmentary matter of 

 equivalent function. This would demand extreme scepticism. 



Later researches have shown that the Spirilla only approach the 

 light when the drop also contains the larger bacteria mentioned above. 

 The latter appeared constantly, although in very small numbers, at 

 the illuminated spot, before the accumulation of the Spirilla began. 

 On examining these bacteria with a high power and a good light, it 

 was seen that they were of a greenish colour, but less intense, how- 

 ever, than that of most chlorophyll-grains of the same size. The 

 aixthor gives them the name of Bacterium chlorinum. They are not 

 identical with B. viride and Bacillus virens of Van Tieghem, which 

 are motionless forms. Bacterium chlorinum has, in a high degree, the 

 tendency of accumulating in the light, but only when oxygen is 

 absent. It is a property it shares with some other green micro- 

 organisms, for instance, with Paramecium hursaria. 



These results make it very probable that the accumulation of 

 Spirilla, cocci, and bacteria, in the light, described at first, was the 

 consequence of the disengagement of oxygen produced by the Bac- 

 terium chlorinum assembled in the illuminated spot. This explanation, 

 however, only seems acceptable on the supposition that the Spirilla 

 only required very little oxygen, much less than the ordinary bacteria 

 of putrefaction, although they are much smaller. 



To verify this supposition, the author has examined the behaviour 

 of Spirilla under different tensions of oxygen. He found that in 

 hydrogen gas as free as possible from oxygen, and even under a plate 

 of glass with hermetically closed edges, they move rapidly many 

 hours after the motion of the bacteria of putrefaction has ceased. 

 Covered with a piece of glass, the Spnrilla do not accumulate, like 

 Bacterium termo, at the very edges of the cover, but at some distance 

 under the glass. If the tension of the oxygen diminishes in the gas- 

 chamber this distance decreases ; if the tension increases the Spirilla 

 retire further. Similar phenomena are observable under the glass 

 cover, around air-bubbles, and green vegetable cells, living and 

 exposed to the light. When the latter are strongly illuminated the 

 zone of Spirilla ceases at a certain distance from them, parallel to 

 their surface ; it approaches when the luminous intensity diminishes, 

 and vice versa. 



There is therefore no doubt that the tension of oxygen most 

 favourable for Spirilla is not much lower than for Bacterium termo. 

 It is certainly less than 150 mm. Hg., and maybe considerably less. 

 The Spirilla re-act at relatively very slight variations of the tension 

 of oxygen. In these respects they behave like certain Flagellala (i. e. 

 Monas termo) and Ciliata {Glaucoma scintillans), which develope by 

 preference in putrefying liquids. 



Vibrios — which, according to the author, cannot be strictly 



