IQ2 IMMUNE SERA 



cultures are then washed off with normal salt solu- 

 tion, using from one to several cc. for each culture. 

 These suspensions are next heated to 55 C. in 

 order to kill the bacteria, and are then standard- 

 ized. By this is meant determining the number of 

 organisms per cc., for Wright always used definite 

 numbers of bacteria in his inoculations. This 

 standardization is readily accomplished by means 

 of the method devised by Wright, which is as fol- 

 lows : From a finger prick a drop of blood is sucked 

 up in a capillary tube to a mark made at any con- 

 venient point with a wax pencil. Next an equal 

 amount of the bacterial suspension is drawn into 

 the tube, allowing a tiny air-bubble to intervene. 

 The two fluids are then expelled on a glass slide, 

 and thoroughly mixed by sucking back and forth 

 a number of times. After this has been done the 

 mixture is spread in the ordinary way of making 

 blood smears. If these blood smears, after stain- 

 ing, are examined with a microscope having a ruled 

 eye-piece, it is a simple matter to determine the 

 ratio of bacteria to blood cells. Taking the red 

 blood cells as 5,000 million per cc., one calculates 

 the number of bacteria per cc. In practice it is 

 advisable to so dilute the bacterial suspension that 

 the dose to be injected is contained in about one 

 cc. of fluid. Finally J per cent, of carbolic acid 

 is added as a preservative. Such a suspension is a 

 " bacterial vaccine." It goes without saying that 



