430 PHYSIOLOGY OF GROWTH. 



whose effect on the bacterium it is desired to test. The capillary, 

 about 6 mm. in length and O06 mm. in diameter, is put into the 

 fluid and filled bj partial exhaustion under the air pump till the 

 air space left at the sealed end is reduced to a length of say 3 mm. 

 For investigation we employ the following fluids : 



(1) Pea infusion prepared as described, in which the bacteria 

 have been killed by boiling, and which has been re-saturated with 

 air by shaking ; 



(2) 1 p.c. meat extract, prepared by dissolving Oo gr. of extract 

 of the ordinary consistency in 50 gr. of water ; 



(3) 2 p.c. solution of Potassium chloride ; 



(4) 2 p.c. solution of Potassium chloride to which has been added 

 5 p.c. Citric acid. 



Results : 



(1) Only isolated bacteria penetrate into the capillaries. 



(2) In the course of a short time (a few minutes) very many 

 bacteria enter the capillary. Numerous swarmers also collect in 

 the neighbourhood of the mouth of the capillary. Gradually, as 

 diffusion from the capillary takes place, these latter bacteria dis- 

 tribute themselves again, and those in the tube, owing to want of 

 Oxygen, in part advance to the air bubble, here gradually forming 

 a second and very close aggregation. 



(3) As 2. 



(4) Isolated individuals penetrate into the capillary, and perish. 

 Most of them rebound from the mouth of the capillary, since the 

 Citric acid exerts a repulsive action on them. 



In making the experiments, the microscope must be free from 

 vibrations. Currents in the bacteria-containing drop are to be 

 prevented as far as possible, and the capillaries containing the 

 fluids under investigation must, before being pushed up to the 

 drop containing the swarmers, be rinsed on the outside by being 

 rapidly passed through water. 



In order to prove that lower organisms are galvanotropic, it is 

 very convenient to work with Paramecium, e.g. Paramecium 

 aurelia. 5 This infusorian, elongated in form, contains a 

 nucleus and two alternately pulsating vacuoles, and is provided 

 with numerous cilia. If a handful of hay is treated in a large 

 vessel with pond water, then in the course of eight to fourteen 

 days there develop in the fluid, besides bacteria, so many Para- 

 mecia that it appears milky. 



We now put together the following apparatus. The wires 



