Bacteriology is essentially a practical study, and even the elements of its technique can only be taught by personal instruction in the laboratory. This is a self-evident proposition that needs no emphasis, yet I venture to believe that the former collection of tried and proved methods has already been of some utility, not only to the student in the absence of his teacher, but also to isolated workers in laboratories far removed from centres of instruction, reminding them of forgotten details in methods already acquired. If this assumption is based on fact no further apology is needed for the present revised edition in which the changes are chiefly in the nature of additions—rendered necessary by the introduction of new methods during recent years.

I take this opportunity of expressing my deep sense of obligation to my confrÈre in the Physiological Department of our medical school—Mr. J. H. Ryffel, B. C., B. Sc.—who has revised those pages dealing with the analysis of the metabolic products of bacterial life; to successive colleagues in the Bacteriological Department of Guy's Hospital, for their ready co-operation in working out or in testing new methods; and finally to my Chief Laboratory Assistant, Mr. J. C. Turner whose assistance and experience have been of the utmost value to me in the preparation of this volume. I have also to thank Mrs. Constant Ponder for many of the new line drawings and for redrawing a number of the original cuts.