lately free from atmospheric air, and into which, therefore, no germ or organic bodies could have been brought by the air"; the latter insisting that only through the entrance of such living organisms could the changes in question take place. The discussion lasted several years, and to-day presents many interesting details, but it may suffice to state that it was ended by Pasteur's demonstration that if the neck of a flask was drawn out into a fine tube and bent into a double curve and the flask then heated by boiling, no decomposition occurred. The flask was open to the atmospheric air, but the microorganisms of the air were arrested by the drop of water of condensation, in the lower point of the curved neck. This demonstration, with the later work of Cohn on spores and of Tyndall on floating matter in the air, disposed of the doctrine of spontaneous generation and led to the universal acceptance of Harvey's law Omne vivum ex ovo, or as it was modified, Omne vivum ex vivo.
It is not surprising that Pasteur at this time foresaw the possibilities in the study of the etiology of the infectious diseases. The process of fermentation, due to living microorganisms, and beginning with a period of apparent inactivity, passing on to a stage of very evident activity and finally sinking gradually into quiescence, was analogous to the period of incubation, the stage of active manifestations and the gradual defervescence of an infectious disease. Also the specificity of the ferments was evidently suggestive of the specific etiology of disease, and altogether we see from several of Pasteur's statements at this time that the relation of microscopic organisms to disease occupied his mind. Thus in a letter to his father, in 1860, he expressed the hope that he may, "bring a little stone to the frail and ill-assured edifice of our knowledge of those deep mysteries of Life and Death where all our intellects have so lamentably failed" and in 1863, after an audience with Napoleon III., he writes, "I assured the Emperor that all my ambition was to arrive at the knowledge of the causes of putrid and contagious diseases."
And now with that peculiar trick of coincidence that is so surprising in the course of culture and inquiry, we find that about this time bacteriology began to make advances along three general lines of study: (1) The etiology of the acute infectious diseases; (2) the prevention of infection, and (3) the achievement of cure or immunity by vaccination. In the first and third of these, Pasteur played a prominent part and it was his work on fermentation which suggested the second to Lister. Pasteur's entrance into the field of etiology and the results he there accomplished form one of the most interesting phases of the history of science and its outcome, a matter of the greatest economic importance to France. The opportunity to study an infectious disease was offered by an epidemic of a mysterious disease which was ruining