PREFACE TO THE ENGLISH EDITION 



NOT with immunity have nations lived, for 

 centuries separate, the one from the other. 

 Their minds have developed along separate 

 lines, and given to each a characteristic turn of 

 thought, and a logic almost individual. 



This individuality of each nation is articu- 

 lated not only into their literature; it is found 

 also in their scientific productions. Without 

 exaggeration it might be said, England sees 

 everything in the form of a mechanism, Germany 

 in that of a formula, whereas France prefers to 

 give its ideas as reasonings. This diversity is 

 not without its advantages, for the unity of 

 scientific truth is like a diamond which gains by 

 being illuminated by every facet. But this 

 advantage only exists where each nation, true 

 to its own genius, takes the trouble to look 

 abroad. This will always be to its profit. 



It would take too long to tell all the benefits 

 which French physicists have derived from 

 scientific intercourse with those of England. In 



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