VI TRANSLATOR'S NOTE 



its technical terms are known even to English scientific 

 men only in their French form. For these I have 

 frequently used periphrases, remembering that many 

 of my readers may not be experts. I must acknow- 

 ledge gratefully the continued assistance of my sister, 

 Mrs. Chalmers Mitchell, not only in making the actual 

 translation, but in revising it for the press. 



EEIC PEITCHARD. 



14, Cromwell Place, S.W. 

 August, 1895. 



