3 i2 HERMANN VON HELMHOLTZ 



solution, producing on the one hand an equivalent chemical 

 decomposition of the solution, and on the other an alteration of 

 concentration at the electrodes. This alteration was continuously 

 reversed, since, where the current weakened the solution, the 

 excess of water was converted into vapour and removed, while 

 at the points where the solution became more concentrated, the 

 vapour was again condensed. If the water is agitated with the 

 dissolved salt, the temperature being kept constant by an 

 appropriate addition of heat, then with weak currents the whole 

 process may be regarded as a reversible cycle, and the sum of 

 the work gained and lost vanishes ; the theoretical conclusions 

 thence deduced agreed satisfactorily with the experimental data 

 previously obtained. 



At the close of the year 1877, Richard Helmholtz left the 

 Polytechnic at Munich, where he had been studying for four 

 years; and Helmholtz had the satisfaction of seeing his son 

 established in Krauss's locomotive factory, where in 1880 he 

 became Head of the Bureau for Construction, and in 1887 Head 

 Engineer. 



Helmholtz now became more and more occupied with official 

 business. 



The time was fast approaching for the delivery of the great 

 Address which he was to give on August 3, 1878, for the Com- 

 memoration of the Berlin University, and in which he contem- 

 plated a free and untrammelled statement of his philosophical 

 creed. He hesitated for some time as to its title. 



4 The title must come last/ he writes to his wife. ' I have not 

 yet decided it. Perhaps "What is Reality?" or "All transitory 

 things are but parables ", or " An Appeal to the Ultimate ", or 

 perhaps merely " Principles of Perception ".' 



In the end he chose the title ' The Facts that underlie Per- 

 ception ', after his wife had written to him, ' I fear that the Ultimate 

 would be an unknown aim to many/ After delivering this most 

 beautiful and most significant of all his lectures, the contents of 

 which have been already outlined, he announces on August 4 

 to his wife : 



'I knew it could not be to the taste of the majority. It 

 contained new ideas that were bound to puzzle them . . . not of 

 course Zeller, du Bois, Kronecker, and kindred spirits. But 

 I said to myself that if I had to work, it should be at something 



