I 



PREFACE TO THE SECOND EDITION 



The rapid sale and early exhaustion of the first edition, both in 

 England and America, give an opportunity for the revision of 

 the whole work and the inclusion of fresh matter that has since 

 been published. Owing to kindly criticism, with contributed 

 facts and suggestions Irom professional friends in various coun- 

 tries, I have been enabled to add much further knowledge, 

 both in theory and practice, of the remarkable development of 

 bacterial treatment of sewage up to the present time. It has 

 not been necessary to alter the statement of principles, such 

 as those of the stages of purification, which are now almost 

 universally acknowledged. 



By condensation of parts of the earlier edition I have found 

 room for much fresh matter, as the aim has been to convey 

 within a small space as much information as possible. 



I have to thank various firms who have lent blocks, and the 

 Main Drainage Committee of the London County Council, who, 

 through Dr. Clowes, have furnished the plates of bacteria to 

 illustrate my accounts of their useful work. 



Samuel Rideal. 



28, Victoria Street, S.W. 

 June, 1 90 1. 



