ON THE NATURE OF TOXINS 39 



cases the task is a difficult one. This is not the case with the 

 non-specific toxins. It is true that in a few isolated instances we 

 are able to increase slightly the resistance of an animal to the 

 simple chemical poisons (e.g., to alkaloids such as morphine), but 

 these apparent exceptions hardly interfere with the utility of the 

 general rule. Further, and more important, an animal immunized 

 to the action of a toxin is also protected against the pathogenic action 

 of the bacterium which produces it, and vice versa. Thus an animal 

 which has been rendered immune to the toxin of tetanus by re- 

 peated injections of that substance is also immune to infection 

 with the living cultures of the bacillus, and an animal which has 

 successfully survived an infection with the tetanus bacillus is 

 thereby rendered in some degree immune to the action of tetanus 

 toxin. This method of immunization with the bacterial toxins 

 (the so-called " chemical vaccination ") is of the utmost impor- 

 tance in practice. It was introduced by Smith and Salmon, who 

 showed that it was possible to immunize pigeons against living 

 cultures of the hog-cholera bacillus by means of the sterilized 

 products of that organism. 



When this method is applicable it supplies us with a test as to 

 the specificity of a toxic substance which we have isolated from a 

 culture of a bacterium, or from the organs of an animal which 

 has been killed by an infection. The substance must be poisonous 

 for animals which are susceptible to the infection in question, and 

 it must be harmless to animals which have been immunized to 

 the organism ; on the other hand, it must immunize animals both 

 to its own action and to that of the bacterium when injected in 

 a living state. These conditions are never fulfilled by the non- 

 specific toxins. 



There are a few apparent exceptions to this rule, but they fail 

 to stand investigation, being based on the fact that it is easier to 

 render an animal refractory to a living organism than to its toxin. 

 Thus an animal which has been injected with the filtered products 

 of certain organisms may be rendered immune to infection with 

 those organisms, but remain as susceptible as before to their 

 toxins. But this is due to the fact that the animal has been 

 immunized but partially ; if the process be carried further the 

 animal will be rendered refractory to both. 



Again, an animal which has been immunized to the toxin of 

 one bacterium remains as susceptible as before to the action of 

 another toxin or bacterium. A horse which has been immunized 



