If it be found impossible to eliminate all the harmful microbes from the flora of the intestines, those that are refractory may be rendered harmless by appropriate serums. We know already a serum that is specific against the microbe of botulism, an organism capable of exciting serious disturbance if it gain entrance to the alimentary canal. Our inmost convictions assure us that life is too short, and since the remotest ages attempts have been made to prolong it. I need hardly mention the quest of the Middle Ages for an elixir of life, but many thoughtful men have occupied themselves with the problem. Descartes, who was deeply interested in the subject, believed himself to have found a mode of lengthening human life. Bacon published a tract on life and death, and in it gave advice as to how old age might be reached; blood-letting and the use of saltpetre were parts of his specific. One of the oldest methods in the world consisted in bringing old men in contact with the bodies of young girls. David, King of Israel, employed this method, which at a much later period came into fashion. Eighteenth-century quacks proclaimed a number of specifics, among which was the “holy water” of Saint Germain, an infusion of senna, merely purgative in its effects. It is certain that some of the medicines used for the purpose, by emptying the large intestine, decreased the bacterial flora, and so checked the formation of the poisons that are harmful to the higher elements. Hufeland,[309] a well-known German professor, published towards the end of the eighteenth century, a work called “La Macrobiotique”; or, “the Art of Prolonging Human Life.” This treatise had a great vogue in its day, and contained many interesting and just observations. Besides advocating cleanliness and moderation, Hufeland advised that “we should use vegetable rather than animal food, as animal food was more liable to putrefaction, whilst vegetable substances contained an acid principle that retarded our mortal enemy, putrefaction.”[310] Here the physician of a day long past anticipated one of the discoveries of modern science. In our time scientific men have not ceased to concern themselves with the prolongation of human life. Professor Pflüger, of Bonn, one of the most distinguished of living physiologists, has published an essay[311] in which he gave the results of his inquiries into this subject. He first stated that investigations into the habits of those who had attained a great age did not give information sufficiently exact. Pflüger laid stress on the means of avoiding infectious maladies, and summed up as follows: “Finally, I can do no better than to associate myself with the advice given in all the treatises on the prolonging of life: avoid the things that are harmful and be moderate in all things.”