216 



IMMUNOLOGY 



immune sera for tlie latter possessed instead a charge-elevating ef- 

 fect. Since the dysentery bacilli possessed a low charge (3 to 5 

 millivolts) while the other bacteria mentioned possessed high 

 charges (23 to 40 millivolts), Shibley concluded that the effect of 

 specific immune serum on its respective antigen is to bring the 

 charge to a common potential level (8-14 millivolts). This is due 

 to the fact that while the antigens differ as to charge and chemical 

 constitution, when they unite with antibody they are alike in that 

 they are coated or filmed with antibody globulin. Since the latter 

 becomes denatured when specifically adsorbed, all of the particles 

 of filmed antigen behave as particles of denatured protein. 



c.cc 



±zS, 



--CJ 



Fig. 12. — Measurement of cohesive force of bacteria. 



T, Torsion balance to measure pull necessary to disrupt films (description by 

 Northrop and De Kruif). C. G., Cover glass. F, Film of bacteria. S, Slide. 

 G-, Glass container. 



Techniques. — Measuring Cohesive Force. — In order to obtain a 

 rough measure of the cohesive force (stickiness) of bacteria, North- 

 rop and DeKruif introduced an interesting technique. This is de- 

 scribed by Northrop as follows : "It occurred to the writer that it 

 might be possible to measure this sticking or cohesive force by de- 

 termining the force required to separate two films of the suspen- 

 sion. This turned out to be the case. The measurement was made 

 by coating two pieces of glass with a thick smear of the suspension. 

 The glass was warmed slightly in order to cause tlie particles to 

 adhere to it, and the two films were allowed to rest together in the 



