From 1894 onwards, the investigations pursued simultaneously at the Paris Natural History Museum, by Phisalix and Bertrand, upon viper-venom, and at the Paris Pasteur Institute by myself, upon that of the cobra, and subsequently upon other venoms of various origins, led to much more definite results. These investigations show, on the one hand, that by vaccinating guinea-pigs or rabbits, and taking certain precautions, it is possible to confer upon these small animals a really strong immunity to venom; on the other hand, that animals vaccinated against cobra-venom are perfectly immune to doses of viper-venom or that of other snakes (_Bungarus_, _Cerastes_, _Naja haje_, _Pseudechis_) certainly lethal to the controls; and lastly, that _the serum of the vaccinated animals contains antitoxic substances capable of transmitting the immunity to other animals_.[97] According to Phisalix and Bertrand, who, as we have stated, experimented only with viper-venom, the best method of vaccinating the guinea-pig consists in inoculating a dose of 0·4 milligramme of this venom heated for five minutes at 75° C., and, forty-eight hours afterwards, the same dose of non-heated venom. The latter is always lethal to the control guinea-pigs in from six to eight hours. Vaccination against cobra-venom, which is much more toxic, is most surely effected by the method recommended by me, which consists in at first injecting small doses of this venom mixed with an equal quantity of a 1 per cent. solution of hypochlorite of lime. By degrees the quantity of venom is increased and that of the hypochlorite progressively diminished, and the injections are repeated every three or four days, while attentively following the variations in the weight of the animals. The inoculations are suspended as soon as emaciation supervenes, and resumed when the weight becomes normal again. After four injections of chloridated venom the chloride is omitted, and a direct inoculation made with one-half the minimal lethal dose of pure venom; then, three or four days afterwards, the injection is increased to three-fourths of the minimal lethal dose; and finally, after the lapse of another three or four days, a lethal dose is injected. If the animals prove resistant, the vaccination can thenceforth be pushed on rapidly, and the quantity of venom injected each time can be increased, testing the susceptibility of the organism by the variations in weight. As a rule, three months are necessary for the vaccination of a rabbit against twenty lethal doses. In six months we can succeed in making it very easily withstand 100 lethal doses. The serum of rabbits thus treated soon, _i.e._, after they have received from five to six lethal doses, exhibits antitoxic properties _in vitro_; these, however, are not very pronounced until after prolonged treatment. They gradually become just as intense as those observed in the case of animals vaccinated against diphtheria or tetanus.