﻿IX, B, 4 Barber: The Pipette Method 341 



a ring of agglutinated vibrios forms instantly and all vibrios 

 which come into this ring are agglutinated at once. If some 

 other vibrio is substituted for cholera, not only does agglutination 

 with the cholera specific serum fail, but the vibrios are attracted 

 by the serum. A good method is first to make a drop of diluted 

 serum very near the bacterial emulsion, then to connect the two ; 

 the serum will flow through the connecting channel into the larger 

 drop. 



This method has been found especially practical in testing out 

 a large number of colonies on a plate. A series of drops of salt 

 solution, peptone water, or other fluid is made on a large cover 

 glass placed over the isolating chamber, and bacteria from various 

 colonies are added to them. Different agglutinating serums may 

 be placed in hanging drops on the same cover or on another 

 cover placed over a second moist chamber. All drops may be 

 tested with a given serum, the pipette washed in a hanging drop 

 of salt solution, and a second serum applied if necessary. If 

 desired, the serum may be used in low dilution in order to elimi- 

 nate negative colonies. Those which give a positive or doubtful 

 test may be further tested by the macroscopical agglutination 

 method. One may dispense with the pipette and test by placing 

 the serum on the under surface of the cover with the loop and 

 connecting it with the emulsion drop to be tested, but the pipette 

 gives more precise results, and is more convenient, especially 

 where a large number of tests are to be made. 



By the same general method a precipitating serum may be 

 added to a hanging drop of the dilution to be tested. The forma- 

 tion of the precipitate may be observed microscopically. Appli- 

 cation of this technique to other serological work as well as to 

 various microchemical tests are possible, but I have had no ex- 

 tended experience in any but the above. 



EXPERIMENTS ON CHEMIOTAXIS 



Experiments of this nature may be carried out with the tech- 

 nique described under serological methods, or by using the 

 capillary tubes in flasks as described on page 346. It would 

 seem possible to test the effects of currents of electricity in 

 hanging drops, with or without organisms. Two pipettes may 

 be used for the two poles by filling them with mercury, as in the 

 technique of inoculating cells. One could insulate one pipette 

 by drawing some nonconducting liquid into the lumen and, by ap- 

 plication of pressure to the pipette, eject the nonconductor and 

 make a mercury connection. If desired, this process may be 

 conducted with both pipettes in the same animal or living vege- 



