STANDARDISATION OF VACCINES 121 



its freedom from gross contamination, and by means 

 of stained preparations to determine its purity. The 

 cultm-e proving satisfactory, 5 c.c. of a 01 per cent, 

 saline solution are pipetted into the tube or bottle, 

 and the growth emulsified as evenly as possible with 

 the help of a glass rod. The turbid emulsion is 

 transferred to a stout test-tube, containing a number 

 of glass beads ; this is then placed in some form of a 

 mechanical shaker, and agitated thoroughly for about 

 fifteen minutes. 



Standardisation of Vaccines.— The amount of 

 bacterial protoplasm present in every cubic centi- 

 metre of the emulsion is next estimated by weighing 

 or by Wright's method. This latter method is now 

 the one perhaps in most common use. It consists 

 in taking equal volumes of blood from a normal 

 individual and of the bacterial emulsion, mixing 

 thoroughly, spreading in a thin film on a glass 

 slide, fixing, and staining with Leishman's stain, 

 and then with the help of a yV oil-immersion lens 

 enumerating the number of red cells and bacteria 

 respectively in some twenty-five separate " fields" of 

 the microscope. From the number thus recorded an 

 average is struck, and the ratio the red blood bears to 

 the bacteria is estimated. Assuming that normal 

 blood contains 5,000 millions of red cells per cubic 

 centimetre, a simple sum in proportion gives the 

 number of bacteria present in each cubic centimetre 

 of the bacterial emulsion. By the addition of O'l 

 saline solution to each cubic centimetre of the 



