viii PREFACE 



the phenomena are treated in elementary courses 

 without reference to the electron, the fundamental 

 entity. 



One difficulty seems to be that physical science is 

 subdivided, classified, and named until the unity is 

 completely obscured. Of each subdivision the accepted 

 method of instruction assigns certain phenomena to 

 high school or sophomore courses and others to junior 

 work. The results of recent researches are postponed 

 for consideration in graduate courses and " recent" 

 often means since the graduation of the instructor. 

 The selection of material for elementary courses is not 

 always determined by whether or not the phenomena 

 are fundamental and easily comprehensible, for it is 

 largely traditional. For example, the writer remembers 

 discussing a well-known text-book with a high school 

 teacher, who said it was not adapted to his field be- 

 cause it introduced the "Brownian movement." The 

 writer, however, believes that the Brownian movement 

 is capable of simple presentation (cf. pp. 155 and 158) 

 and should be discussed in order that the student may 

 obtain satisfactory concepts upon which to base his 

 future study. 



So far as concerns college instruction one alternative 

 is a general introductory course which cuts across the 

 conventional divisions of subject matter, selecting to 

 emphasize the unity. Nor should this introduction 

 have the finality of the usual elementary course, for it 

 should indicate the evolutionary character of science 

 and lead up to present problems. It should also indi- 

 cate the historical aspects and the social significance 

 of modern science. In material it should be adapted 



