EFFECT OF HIGH PRESSURE. ON BACTERIA 73 



Weak currents act on bacteria presumably in the same way that they 

 do on infusoria and other motile micro-organisms (44). These are ' galvano- 

 tropic ' ; they collect around the kathode (negative pole). If the direction 

 of the current be reversed, the infusoria also turn round 180 degrees, place 

 themselves parallel to the current, and swim towards the other (now 

 negative) pole. Experiments with bacteria have not yet been made, 

 and they would be very difficult on account of the small size of their 

 cells. 



Very numerous and thorough experiments have been made as to the 

 effect of Rontgen rays upon bacteria, and it has been conclusively shown 

 that they are not even inhibitory, let alone fatal to the cells (45). The 

 higher plants behave in the same way, and the hopes that were entertained 

 of being able to disinfect the diseased body by means of Rontgen rays have 

 not been realized. 



High pressure (46) seems to be . absolutely without influence upon 

 bacteria. The processes of putrefaction and alcoholic fermentation go on 

 without interruption under a pressure of from 300 to 500 atmospheres, and 

 the spores of the anthrax bacillus are unchanged after exposure for 

 24 hours to 600 atmospheres. In considering such facts as these it must 

 not be forgotten that the weight supported by a single bacillus (say of 

 5 n by 1 n) is extraordinarily small (even at 500 atmospheres only about 

 80 milligrams), so that even at the bottom of the deepest ocean at a depth 

 of 7,086 metres a coccus of a n diameter would only have to withstand 

 a pressure of 90 milligrams. In the face of these facts we are hardly 

 justified in saying that bacteria are less sensitive to pressure than other 

 organisms, because the behaviour of the larger plants and animals seems to 

 be in this respect incommensurate with that of bacteria. 



The force of gravity exercises upon bacteria no effect comparable with 

 the geotropism of other plants. 



Bacteria, like higher plants and cold-blooded animals, are organisms 

 whose temperature corresponds to that of their environment, falling and rising 

 with the external temperature ; they are poikilothermic (47). The relations 

 of the cell to temperature are expressed by specifying three points on the 

 thermometric scale, the maximum, minimum, and optimum. These three 

 cardinal points hold good for all organisms. The different functions of life 

 are performed best at different temperatures, and the temperatures most 

 favourable to growth, to movement, to fermentative power, to virulence, to 

 sporulation and to germination are probably different in each case, but 

 a general average, coinciding with the most rapid multiplication of the cells, 

 is spoken of as the ' optimum.' For all species these three cardinal points 

 vary slightly within certain limits. 



The following cardinal points are those which hold good for growth. 

 The maximum is that temperature which cannot be exceeded without 



