PREFACE 



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In presenting this work to the judgment of my 

 critics, as the result in part of my studies for some 

 years past, in some respects in outline whilst in 

 others in detail, and more so perhaps than it was 

 my intention at first to make it, two points 

 should be borne in mind. In the first place, it is 

 intended that the chapters which deal with phosphor- 

 escence may with advantage be omitted by those who 

 are not particularly interested in such matters, since 

 they may tend to divert the attention from the 

 real points at issue, if the mind is not specially 

 familiar with physical conceptions. The reader who 

 does not wish to be burdened with detail is there- 

 fore recommended to omit them, at any rate on 

 the first reading, if the book should prove of 

 sufficient interest to demand his perusal for a second 

 time. Those chapters of a purely physical and 

 perhaps somewhat technical nature, which he thus 

 may think it worth his while to pass over, have 

 been introduced to illustrate the way in which 

 the phenomenon which I have called physical 

 metabolism admits of comparison with the highly 



