These new studies are based on the same principles which guided me in my researches on wine, vinegar, and the silkworm disease—­ principles, the applications of which are practically unlimited.  The etiology of contagious diseases may, perhaps, receive from them an unexpected light.

I need not hazard any prediction concerning the advantages likely to accrue to the brewing industry from the adoption of such a process of brewing as my study of the subject has enabled me to devise, and from an application of the novel facts upon which this process is founded.  Time is the best appraiser of scientific work, and I am not unaware that an industrial discovery rarely produces all its fruit in the hands of its first inventor.

I began my researches at Clermont-Ferrand, in the laboratory, and with the help, of my friend M. Duclaux, professor of chemistry at the Faculty of Sciences of that town.  I continued them in Paris, and afterwards at the great brewery of Tourtel Brothers, of Tantonville, which is admitted to be the first in France.  I heartily thank these gentlemen for their extreme kindness.  I owe also a public tribute of gratitude to M. Kuhn, a skillful brewer of Chamalieres, near Clermont-Ferrand, as well as to M. Velten of Marseilles, and to mm. de Tassigny, of Reims, who have placed at my disposal their establishments and their products, with the most praiseworthy eagerness.

L. Pasteur.

Paris, June 1, 1879.

THE PHYSIOLOGICAL THEORY OF FERMENTATION

I. ON THE RELATIONS EXISTING BETWEEN OXYGEN AND YEAST

It is characteristic of science to reduce incessantly the number of unexplained phenomena.  It is observed, for instance, that fleshy fruits are not liable to fermentation so long as their epidermis remains uninjured.  On the other hand, they ferment very readily when they are piled up in heaps more or less open, and immersed in their saccharine juice.  The mass becomes heated and swells; carbonic acid gas is disengaged, and the sugar disappears and is replaced by alcohol.  Now, as to the question of the origin of these spontaneous phenomena, so remarkable in character as well as usefulness for man’s service, modern knowledge has taught us that fermentation is the consequence of a development of vegetable cells the germs of which do not exist in the saccharine juices within fruits; that many varieties of these cellular plants exist, each giving rise to its own particular fermentation.  The principal products of these various fermentations, although resembling each other in their nature, differ in their relative proportions and in the accessory substances that accompany them, a fact which alone is sufficient to account for wide differences in the quality and commercial value of alcoholic beverages.