60 Lectures on Bacteria. [ vi. 



all events, they were those in which the kefir-organism grows best 

 according to our present knowledge, namely, in milk at an air- 

 temperature of 1 5-20 C., and with a supply of atmospheric air. 



The movements also of the Bacteria, as well as their growth 

 and germination, are directly dependent on the conditions of 

 vegetation in the species and forms which are capable of inde- 

 pendent movement. The occurrence and the direction of the 

 motion are specially determined by the influence of nutrient 

 substances, and of oxygen. If a form of this kind, Bacillus 

 subtilis for instance, in the vegetative condition in which it is 

 capable of movement is placed in a drop of nutrient solution on 

 a slide under a cover-glass, the motile rods are seen to collect 

 at once round the margin of the cover-glass where the oxygen 

 of the air has free access. The comparatively few which remain 

 behind in the centre of the drop, and are there cut off from the 

 atmospheric oxygen, become slower in their movements and 

 finally lose them altogether. Aerobiotic forms enclosed in a 

 drop of water in which there is no free oxygen along with 

 Algae containing chlorophyll at first remain motionless. But 

 as soon as the cells containing chlorophyll are induced to give 

 off oxygen under the influence of light, the Bacteria begin to 

 move actively, as Engelmann (31) has shown, and the movement 

 is directed towards the spots where the oxygen is being given 

 off. Here the Bacteria collect, and they may therefore be used 

 as an extremely delicate reagent for the detection of quantities 

 of oxygen of almost inconceivable minuteness. The frequent 

 grouping of aerobiotic forms into films or membranes on the 

 surface of fluids is no doubt partly due to the influence in 

 question determining the direction of the movement. 



While the above-mentioned forms approach as near as pos- 

 sible to the source of the atmospheric oxygen, there are others 

 which, as Engelmann (26) found in the case of a Spirillum, 

 always remain at a certain distance from it, the distance 

 diminishing as the amount of free oxygen diminishes in the air 

 which finds access to the Bacteria. This observation proves the 



