vi Preface 



years have passed, and out of its revelations 

 we have won treasures of knowledge, of in- 

 sight, and practical beneficence to man, which 

 lie far beyond even the dreams of an earlier 

 day. 



While the revision of the book, for this 

 second edition, has been everywhere exten- 

 sive, the greater changes and additions have 

 been made in the sections relating to disease 

 and its prevention. 



The addition of pictures and the enlarged 

 scope of the book, it is hoped, will make it 

 useful to the new generation of readers, 

 whose outlooks for increased efficiency and 

 happiness in life, is curiously interlinked with 

 the performances of these invisible earth 

 neighbors of ours, whose story is here briefly 

 rehearsed. T. M. P, 



