INTBODUCTION. 



IN the year 1850, -when I -was a student in the University of 

 Marburg, it was my privilege to translate for the 'Philosophical 

 Magazine' the celebrated memoirs of Clausius, then just pub- 

 lished, on the Moving Force of Heat. 



In 1851, through the liberal courtesy of the late Professor 

 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-Eaymond 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 Helmholtz and Clausius, 



