"And you hope to conquer other diseases in the same way?" The doctor made a fine nervous gesture. "In science one does not hope; one proves. In every thirty thousand experiments one succeeds. We study diseases here. Each physician has his special line of investigation. We hope for nothing. We simply report what we find." "But you yourself, Doctor Roux, have certainly hopes that diphtheria is almost conquered?" The doctor pursed up his mouth. "The investigations in diphtheria are in just this condition. We have proved at the Pasteur Institute" (Doctor Roux is modest and says 'we,' which means himself and his assistant, M. Versin) "that diphtheria is a toxic disease; that is, that it results from a poison. The microbe of diphtheria does not penetrate throughout the system as in the case of most other microbic diseases. It exists only in the mucus found on the pharynx. This microbe does not cause death, but it secretes a poison which penetrates throughout the body and kills. This being proved, of course the next step is to find what will destroy the poison. "Doctor Behring, working at Berlin, has found that the blood of animals vaccinated for diphtheria gives a therapeutic serum which destroys the diphtheric poison. We are now testing the practical value of the serum at the Institute. This is absolutely our 'last word' on diphtheria." "And as for cholera! What is the last word?" "The _bacillus virgula_ of Doctor Koch is believed, by the great majority of savants, to be the true cholera germ. We are trying here, as experimenters are trying elsewhere, to give immunity to animals against the microbe. It is absolutely all that one can say authoritatively on the cholera." "And the method of vaccination which Doctor Haffkine believes he has discovered?" "It has not been proved yet that it will give immunity. Until we have that proof we neither hope nor fear. We simply work and wait. Doctor Haffkine has, you know, severed his connection with the Pasteur Institute and gone to India to continue his researches." "But he has had faith enough in his method to try its effects on himself, has he not?" "Very true, and so have perhaps a hundred others tried its effects. But that proves nothing." There is a self-repression about these severe statements which has something of the heroic in it. Who would be so glad to announce absolute safeguards against diphtheria and cholera as this man who has risked his life to find them? Yet, until he is sure, he will not even say "hope." I remember the words of Pasteur himself: "To believe that one has found an important scientific fact, to be in a fever to announce it, to compel one's self for days, weeks, sometimes years, to be silent, to force one's self to destroy his own experiments and to announce nothing until he has exhausted all contrary hypotheses--that is hard."