PHYSICS OF STREAMING 



FIG. 8. Ringed preparation of motile organisms between the pole- 

 pieces of an electro-magnet, a = air-bubbles. 



SECTION 19. The Influence of Magnetic Forces on freely motile 



Organisms. 



Both aerobic and anaerobic motile Bacteria, ciliate and flagellate Infu- 

 soria, as well as the spermatozoids of a few plants, were used. Even the 



strongest field exercised no 

 perceptible directive influ- 

 ence, nor was the velocity 

 of movement affected by 

 the entry or exit of the field. 

 This is, however, hardly 

 surprising, for the propulsive 

 force necessary to overcome 

 the resistance of the water 



m ,_ t v_ rrm oiHprah1p PC 



compared with the restrain- 

 ing and directive action exercised by the field. Similarly, no perceptible 

 retardation could be seen after actively motile Infusoria and Bacteria had been 

 moving about in all directions in a strong field for fifteen to thirty minutes. 



If, however, aerobic forms are placed under a ringed cover-slip, as in 

 the figure, the organisms pass to and fro from one air-bubble to the other 

 across the lines of force, and hence are subjected to internal electrical 

 currents altering with each change in the direction of movement. These 

 produce in ten to forty minutes a retardation which may amount to 

 a quarter to three-quarters of the average original velocity. If the air- 

 bubbles are large and the number of organisms limited, an optimal supply 

 of oxygen is assured for two or three hours, and on removing the field 

 the velocity often increases again in half to one hour. In other cases the 

 retardation is permanent, and on lifting the cover-slip no acceleration occurs. 

 These results are best shown by large and small flagellate infusoria. 



When the period of swarming is limited, as in the sperms of the fern, 

 Chara and Vaucheria, these seem to come to rest sooner in a strong field 

 than they would otherwise have done, the difference often amounting to 

 as much as a quarter to one hour. Non-motile elongated bacillus-rods 

 frequently swing slightly or show a tendency to place their long axes 

 parallel to the lines of force in a strong magnetic field. Even slowly moving 

 forms, however, are able to overcome this directive action, and hence to 

 move in all directions, but if they are allowed to come to rest l in the field, 

 around the periphery of the latter though not at the centre, it can fre- 

 quently be noticed that the long axes of the greater number make angles 

 of from o to 45 with the lines of force. The organisms are obviously 

 paramagnetic in water. 



1 In a hanging drop and also in ringed preparations. Most Infusoria burst shortly after all the 

 oxygen has been exhausted. 



