282 XXI. BACTERIA YEAST. 



clots on the surface of the fluid. If now we examine the swar- 

 mers in a drop of fluid under a large cover-glass, it will be seen 

 that the movement soon ceases. It is continued longest around 

 any enclosed air bubbles, and at the edges of the cover-glass. 

 Eound the edge a thick layer of swarmers will soon have collected, 

 cutting off all entrance of air. Ultimately all the swarmers come 

 to rest. If, however, in making the preparation we have enclosed 

 in it a green algal thread, the movement of the swarmers con- 

 tinues around this thread so long as it. is subject to the influence 

 of light. They collect in considerable number round the thread, 

 and if this contains chromatophores confined to special parts, 

 these parts are sought out by the bacteria. The oxygen given 

 out by the chromatophores acts as a stimulus, which causes the 

 movement of the bacteria, and determines even the direction of 

 the movement. For example, in Spirogyra the aggregated bac- 

 teria follow the green band. If the preparation is placed in 

 darkness, the movement ceases round the green cells also ; and 

 it recommences immediately the thread is again exposed to the 

 action of light, whereby it begins to assimilate and give off oxygen. 

 To prevent access of oxygen from the atmosphere a large cover- 

 glass should be used, and this may even be sealed at its edges 

 with paraffin or with cocoa-butter. The swarming stage of bac- 

 teria is, therefore, a very suitable test for oxygen, and it has been 

 made use of in the case of Engelmann's bacteria method, in 

 order to measure the strength of the assimilation of carbon in the 

 different parts of the spectrum, and so in some degree to measure 

 the relative values for this purpose of the various rays which 

 constitute white light. Special micro -spectral objectives can be 

 obtained for the purpose of throwing a small spectrum upon the 

 object-slide under the microscope ; or, if such a micro-spectro- 

 scope is not at our disposal, we can obtain a partial idea of the 

 energy of the assimilation of carbon under the influence of 

 different rays by allowing the light to pass through coloured 

 glass, or coloured fluids, the spectroscopic character of which we 

 have previously determined. 



Culture Methods. The greatest possible attention has been 

 given to culture methods for bacteria, in consequence of the theo- 

 retical and practical importance which they have acquired. These 

 cultures have become an important aid for the identification of 

 bacteria, which can often be more readily distinguished macro- 

 scopically, by the form and method of growth of their colonies v . 



