vi PREFACE TO THE FIFTH EDITION 



are given on the purification of water, and the bacteriology 

 of milk and foods has been revised. In fact, hardly a 

 page remains quite the same as previously, and it is hoped 

 that the text has been made clearer and the contained 

 matter brought up to date. 



My grateful thanks are due to my friends and colleagues, 

 DR. FRANK TAYLOR, upon whom the labour of the revision 

 of the proof-sheets has largely fallen, and MR. J. E. 

 BARNARD, who has furnished some new photo- micrographs. 



R. T. H. 



UNIVERSITY OF LONDON] KING'S] COLLEGE 

 May 1914 



