Undoubtedly the most interesting of Father Kircher's scientific books is his work On the Pest, with some considerations on its origin, mode of distribution, and treatment, which about the middle of the seventeenth century gathered together all the medical theories of the times as to the causation of contagious disease, discussed them with critical judgment and reached conclusions which anticipate much of what is most modern in our present-day medicine. It is this work of Father Kircher's that is now most often referred to, and very deservedly so, because it is one of the classics which represents a landmark in knowledge for all time. It merits a place beside such books as Harvey on the Circulation of Blood, or even Vesalius on Human {128} Anatomy. As we have seen, it is now quoted from by our best recent authorities who attempt seriously to trace the history of the microbic theory of disease, and its conclusions are the result of logical processes and not the mere chance lighting upon truth of a mind that had the theories of the time before it. In it Father Kircher's genius is best exhibited. It has the faults of his too ready credibility; and his desire to discuss all possible phases of the question, even those which are now manifestly absurd, has led him into what prove to be useless digressions. But on the whole it represents very well the first great example of the application of the principle of inductive science to modern medicine. All the known facts and observations are collected and discussed, and then the conclusions are suggested. It is very interesting to trace the development of Father Kircher's ideas with regard to the origin, causation, and communication of disease, because in many points he so clearly anticipates medical knowledge that has only come to be definitely accepted in very recent times. It has often been pointed out that Sir Robert Boyle declared that the processes of fermentation and those which brought about infectious disease, were probably of similar nature, and that the scientist who solved the problem of the cause of fermentation would throw great light on the origin of these diseases. This prophetic remark was absolutely verified when Pasteur, a chemist who had solved the problem of fermentation, also solved {129} the weightier questions connected with human diseases. Before even Boyle, however, Father Kircher had expressed his opinion that disease processes were similar to those of putrefaction. He considered that putrefaction was due to the presence of certain _corpuscula_, as he called them, and these he said were also probably active in the causation of infectious disease.