INTRODUCTORY AND GENERAL 25 



carried out by means of standard loops, each of which will pick 

 up a known amount of growth. (c) In Wright's ingenious 

 method of counting a vaccine a certain amount of the latter is 

 mixed with human blood in definite proportions, and films are 

 prepared and stained. The numbers of red corpuscles and of 

 bacteria in several fields of the microscope are then counted, and 

 (the numbers of red corpuscles in a definite volume of the blood 

 being known) the proportions of the two will permit of the calcu- 

 1 ation of the numbers of the bacteria, (d) Some determine the 

 strength of the vaccine by reference to a permanent standard, 

 usually consisting of a fine suspension of barium sulphate. A 

 strong emulsion of bacteria is prepared and diluted until it 

 matches the standard, (e) The volume of the bacteria in the 

 emulsion may be determined by centrifugalization in a graduated 

 tube, and a certain volume of sediment made up to a certain 

 volume of vaccine. (/) The number of bacteria present in the 

 emulsion may be counted directly by the use of the counting 

 chamber of the hsemocytometer, and this is the method I usually 

 employ. The emulsion is diluted (usually to twenty times its 

 volume) with a dilute solution of methylene blue or other stain, 

 boiled, and a drop placed in the counting chamber and prepared as 

 if it were a blood specimen in which the red corpuscles were to be 

 counted, A ^-inch lens and a high eyepiece are used, and, as a 

 rule, the process presents no difficulty. 



In all cases an addition of a chemical antiseptic is advisable 

 to avoid subsequent contamination. Carbolic acid or lysol (0*25 to 

 0-5 per cent.) are most used; another good plan is to keep a few 

 drops of chloroform at the bottom of the bottle, so that the fluid 

 is always saturated. 



This method is mostly used in plague, cholera, and enteric 

 fever in preventive medicine, and in the treatment of infective 

 processes by Sir Almroth Wright's method in curative medicine. 

 These will be discussed subsequently. 



4. Inoculation with the chemical products or with the toxins 

 of the bacteria, the bodies of the bacteria themselves being 

 removed by filtration or in some other way. This is obviously 

 closely allied to the last method the use of killed cultures. 



It was introduced by Smith and Salmon in the case of hog 

 cholera, and is now chiefly used in the immunization of animals 

 for the production of antitoxic sera. It is considered fully in a 

 subsequent chapter. 



