The above are methods of bacterial vaccination. Salmon and Smith have shown that artificial active immunity can be produced also by (3) _toxin-injection_. They artificially cultivated the hog-cholera bacillus in broth. This broth was then sterilized, the bacilli being killed, but their products remaining. By injecting pigeons with this sterilized broth they made them resistant to subsequent infection by the bacillus itself, thus proving that immunity can be produced by chemical as well as by biological means. The immunity was proportional to the dose of the toxin absorbed. By gradually increasing the dose, it was found practicable to confer immunity, not only against doses of toxin that would otherwise have been fatal, but also against bacterial infection by the particular bacillus used in manufacturing the toxin. =Passive Immunity.=—Behring and Kitasato found that if the toxin (free from the bacilli) of tetanus be injected into an animal in increasing doses until it becomes immune against infection by the bacilli of tetanus, the blood serum of the animal in question injected into white mice confers the same immunity on them. The protection thus conferred is only temporary. Exactly the same procedure has been adopted for diphtheria, and it is now found that by injecting _anti-diphtheritic serum_ into children who are exposed to the infection of diphtheria, they can for several weeks be prevented from developing the disease. This is of great practical importance, as meanwhile the source of infection can have been removed. Furthermore, the _protective serum is also curative_, and by its means diphtheria, if early treated, can be reduced from a dangerous to an insignificant disease. Various =theories= have been propounded =to explain immunity=. Pasteur supposed that the special pabulum or food of the bacillus of the given disease became exhausted; but this does not fit in with the immunity that can be produced by toxins and anti-toxins. Chauveau supposed that certain bacterial products are retained in the body, rendering it unsuitable for further growth of the particular bacillus; just as more than 14 per cent. of alcohol in a saccharine solution prevents further fermentation. This does not explain all the facts. Metschnikoff concluded that the fight of the leucocytes and phagocytes of the body against weaker bacilli, gave the power of fighting and overcoming a more virulent bacilli, and that these properties of the cells were transmitted to later generations of body cells. This theory fails to explain the acquired immunity against toxins as well as against bacteria which occurs. The discovery that the blood and other normal tissue fluids possess some power of destroying bacilli has relegated the phagocytal theory to a secondary position.