The author is indebted to many friends for their assistance in getting the book together, and would specially mention Dr. H. B. Hutchinson, Bacteriologist, Rothamsted Experimental Station, for assistance in connection with the bacteriology of fermented milks; Mr. Thomas Douglas, of Wimbledon, who has assisted with the chemistry of the subject; Mr. S. Javrilovitch, of Belgrade, Servia, for local information and illustrations; Dr. Otokar Laxa, Bacteriologist, of Prague, Bohemia, for general assistance; the editor of _Bacteriotherapy_, New York, U.S.A., for the use of the group of illustrations 30-44; the publishers of the _Centralblatt für Bakteriologie_, Jena, for the group of illustrations 14-29; and many others, some of whom are referred to in the text. CONTENTS CHAPTER PAGE I.—INTRODUCTORY—HISTORICAL 1 II.—FERMENTED MILKS 15 III.—THE CHEMISTRY OF MILK 47 IV.—HANDLING OF MILK 68 V.—THE BACTERIOLOGY OF FERMENTED OR SOURED MILK 84 VI.—THE PREPARATION OF SOURED MILK IN THE HOUSE 125 VII.—THE PREPARATION OF SOURED MILK IN THE DAIRY 139 VIII.—SOURED MILK IN HEALTH AND DISEASE 151 INDEX 165 ILLUSTRATIONS FIG. PAGE THE OLDEST WOMAN IN THE WORLD AND HER SON _Frontispiece_ THE PASS OF BUKOVA 2 KABYLES SOURING MILK 4 THE HANDLING OF MILK IN THE PYRENEES 8 THE CONSTITUENTS OF MILK 48 1.—MICRO-PHOTOGRAPH OF A DROP OF WHOLE MILK 58 2.—MICRO-PHOTOGRAPH OF SEPARATED MILK 58 3.—MICRO-PHOTOGRAPH OF CREAM 58 4.—PHOTOGRAPH OF TWO PETRI DISHES, WHICH HAVE BEEN INOCULATED WITH ORDINARY MILK 60 5.—THE CREAMOMETER 62 6.—TESTING-GLASS FOR EXTRANEOUS MATTER IN MILK 62 7.—LACTOMETER AND TEST-GLASS 64 8.—PASTEURISER 78 9.—CONTINUOUS APPARATUS FOR THE PRODUCTION OF LARGE QUANTITIES OF SOURED MILK 82 10.—A MILK-FILLING APPARATUS 90 11.—SECTION THROUGH A KEPHIR GRAIN 94 12.—_Streptococcus lacticus_ (GROTENFELDT) GROWING ON LACTOSE-AGAR, STAINED BY GRAM'S METHOD 96 13.—PHOTO-MICROGRAPH OF PREPARATION FROM ARMENIAN SOURED MILK (MATZOON) 106