Some reference has already been made to the empirical knowledge of the earlier ages. Even Pliny’s often quoted “_Palam est naturam (farinæ) acore fermentari_” is merely a summary of the result of observation. Noah’s wine making, the leaven[3] of the Jews and such like may be left to special history. The word _fermentum_ as used by the alchemists has no very definite meaning; in general their explanation is to the effect that by means of the ferment a purifying and refining process is set in action—and hence many efforts were made to discover a general ferment by whose instrumentality it would become possible among other things, to transform the baser metals into gold. For this reason they often use the word _fermentum_ to indicate the anxiously sought “philosopher’s stone.”[4] The indefinite character of the word is mentioned by Petrus Bonus of Ferrara (1345): “_Apud philosophos fermentum dupliciter videtur dici: uno modo ipse lapis philosophorum ex suis elementis compositus et completus, in comparatione ad metalla; alio modo illud quod est perficiens lapidem et ipsum complens_,” and Raymond Lull’s definition, “_Fili, fermentum est corpus perfectum, subtiliatum et alteratum per potestatem convertentium_,” has the predicate so indefinite as to give no real information. We add another quotation from the same author merely to show further the jargon these men of learning were accustomed to use. He writes “_Fili, præparatio istius est, quod illud sit transactum primo per naturæ principalia controvertentia, antequam de isto facias fermentationem, quia illud fiat principio pulvis calcinatus per coagulationem et quarto sublimatus per separationem._” George Ripley’s consideration of the subject calls for no special notice, but the views of Basilius Valentinus who wrote in the latter half of the fifteenth century will be found more interesting. He held fermentation to be a purification by means of which the spirit of wine that already existed in a fluid was put in condition to act, unfermented beer being dead, “because existing impurities prevent the spirit from doing its work. Yeast induces in beer an internal quickening that advances of itself and results in a division and segregation of the clear and muddy elements, and after this separation _puri ab impuro_ the spirit can accomplish its duty successfully, as appears from the subsequent power of the liquor to produce intoxication.” Valentine is the last in the series of scholars who though belonging chronologically to a previous epoch must from the nature and relations of their inquiries be reckoned as belonging to the new era. It is not in the history of progress as in that of politics where two adjacent periods can be sharply defined and their limits assigned to exact dates. Progress goes on gradually, modifying or adding to what has already existed, and we do not clearly notice the transformation until it is complete or at least far advanced. So it was in this case.