INTRODUCTION V 



vince the child by the same facts that have convinced 

 his elders. 



The current notion has been, " a little book for a little 

 child." To me it seems so true as to be almost axiomatic, 

 that the younger the child the more often must a fact be 

 told and the more fully must it be elaborated. The 

 capacity to profit by generalized statements comes only 

 with age. For this reason, in the present series, even on 

 the subjects of alcohol and narcotics, dogmatic assertion 

 and the easy moral have been avoided. Treatment of 

 subjects by this method necessarily increases the size 

 of the text somewhat, but it also rouses and holds the 

 interest of the reader. 



Although I have thus planned the series myself, the 

 work of writing the separate volumes has been done by 

 others. And it is but just to the author of each book to 

 say that in preparing the facts for presentation no pains 

 has been spared to secure acquaintance at first-hand 

 with the work of the original investigators upon whose 

 authority the facts rest. An illustration of this is found 

 in the study of pure water as given later in the series 

 when typhoid fever is under discussion. The author not 

 only visited the experiment station in Lawrence, Massa- 

 chusetts, where more scientific work has been done in 

 sewage filtration than elsewhere in the United States, 

 but also made careful study of water conditions in New 

 York, London, Pittsburg, and other important cities. 



