BY DEAD CULTURES. 467 



cases a high degree of immunity against infection by a 

 given microbe may be developed by repeated and gradually 

 increasing doses of the dead cultures, the cultures being 

 killed sometimes by heat, sometimes by exposure to the 

 vapour of chloroform. Some consider that in this method 

 only the intracellular toxic substances of the organism are 

 introduced when the cultures have been taken from the 

 surface of a solid medium, such as agar, but as the surface 

 is moist, some of the extracellular products must be present 

 also. The cultures when dead produce, of course, less 

 effect than when living, and this method may be con- 

 veniently used in the initial stages of active immunisation, 

 to be afterwards followed by injections of the living cultures. 

 The method has been extensively used by Pfeiffer and 

 others in the production of a high degree of immunity in 

 guinea-pigs against the typhoid, cholera, and other organisms. 



3. Immunity by the Separated Bacterial Products or 

 Toxines. The organisms in a virulent condition are grown 

 in a fluid medium for a certain time, and the fluid is then 

 filtered through a Chamberland or other porcelain filter. 

 The filtrate contains the toxines, and it may be used 

 unaltered, or may be reduced in bulk by evaporation, or may 

 be evaporated to dryness. The process of immunisation 

 by the toxine is started by small non-lethal doses of the 

 strong toxine, or by larger doses of toxine the power of which 

 has been weakened by various methods (vide infra}. After- 

 wards the doses are gradually increased. Immunity pro- 

 duced in this way is effective not only against the toxine, 

 but also against large doses of the virulent organism in a 

 living condition. This method was carried out with a 

 great degree of success in the case of diphtheria, tetanus, 

 malignant oedema, etc. It appears capable of very general 

 application, though, in the case of many organisms, it is 

 difficult to get a very active toxine from the filtered cultures. 

 It has also been applied in the case of snake poisons by 

 Calmette and Fraser, and a high degree of immunity has 

 been produced. 



Immunity may also be obtained by means of certain 



