INTRODUCTION. 



In the year 1850, when I was a student in the Univer- 

 sity of Marburg, it was my privilege to translate for 

 the ' Philosophical Magazine ' the celebrated memoirs of 

 Clausius, then just published, on the Moving Force of 

 Heat. 



In 1851, through the liberal courtesy of the late Pro- 

 fessor Magnus, I was enabled to pursue my scientific 

 labours in his laboratory in Berlin. One evening during 

 my residence there my friend Dr. Du Bois-Kaymond put 

 a pamphlet into my hands, remarking that it was ' the 

 production of the first head in Europe since the death of 

 Jacobi,' and that it ought to be translated into English. 

 Soon after my return to England I translated the essay and 

 published it in the ' Scientific Memoirs,' then brought out 

 under the joint-editorship of Huxley, Henfrey, Francis, 

 and myself. 



This essay, which was communicated in 1847 to the 

 Physical Society of Berlin, has become sufficiently famous 

 since. It was entitled ' Die Erhaltung der Kraft,' and 

 its author was Helmholtz, originally Military Physician 

 in the Prussian service, afterwards Professor of Physiology 

 in the Universities of Konigsberg and Heidelberg, and 

 now Professor of Physics in the University of Berlin. 



Brought thus face to face with the great generalisation 

 of the Conservation of Energy, I sought, to the best of 

 my ability, to master it by independent thought in all its 

 physical details. I could not forget my indebtedness to 



