PROFESSOR AT HEIDELBERG 199 



woman, and Anna went to her several times for long visits 

 to Paris and England, where she imbibed the best sides of 

 French and English manners and customs. I must confess 

 that I rather avoided than sought Anna v. Mohl last summer, 

 for I felt that a girl like her would be dangerous for me, and 

 I should never have presumed, as a widower with two children, 

 and no longer in my first youth, to seek the hand of so young 

 a lady, who had every qualification for playing a prominent 

 part in society. However, it all came about very quickly, 

 and now I can once more face the future happily. The 

 wedding is to be at Whitsuntide/ 



In the Easter vacation Helmholtz went to England to give 

 two lectures on 'The Physiological Theory of Music', but found 

 he had to deliver a third without any preparation, since Bence 

 Jones and Faraday insisted on his giving an evening discourse 

 on the Conservation of Energy. In this lecture, delivered April 

 12, 1861, * On the Application of the Law of the Conservation 

 of Force to Organic Nature/ he first, as in all his earlier 

 lectures, gives an account of the principle of the conservation 

 of force, which, with Rankine, he prefers (since it bears no 

 relation to amount of force) to term the conservation of energy, 

 and which he designates as the most important advance 

 of science in the century because it embraces all laws of 

 physics and chemistry. He now proceeds to apply this 

 law to organic nature ; he points out that continuation of life 

 is bound up with continued supply of means of sustenance, 

 which after complete digestion pass into the blood, are slowly 

 consumed in the lungs, and finally produce almost the same 

 compounds with the oxygen of the air, as those which 

 would be produced by burning the food in an open fire. 

 Now since the amount of heat produced by oxidation is 

 independent of the time occupied in consumption and of 

 the intermediate stages, it can be calculated from the 

 mass of the materials consumed, how much heat, or its 

 equivalent work, can be produced by any animal body 

 experiments that involve great difficulties. On a later occa- 

 sion, when a criticism of some scientific book was demanded 

 from him, he pointed out that the difficulty which arises in con- 

 sidering these very important and very recondite physiological 

 problems (the question of heat production in animal bodies, 



