210 HERMANN VON HELMHOLTZ 



discovered by its aid should be freely communicated -to his 

 students and his fellow citizens; he always appreciated the 

 fact that lecturing compelled him to test each isolated pro- 

 position strictly, to formulate each conclusion correctly, and, 

 since he could only assume a limited amount of previous know- 

 ledge in his hearers, to state the evidence for the views he was 

 maintaining in as simple a manner as possible. His audience 

 took the place of his friends, whom he always imagined as 

 present at his scientific lectures. ' I always pictured the most 

 intelligent of my friends before me, as my conscience ; I asked 

 myself if they would sanction what I was saying. They haunted 

 me as the embodiment of the scientific spirit of an ideal 

 humanity, and set my standard/ 



' As a student in Heidelberg/ says Engelmann, ' I followed his 

 lectures on physiology, and the public lectures on the general 

 results of natural science, which he gave every winter at that 

 time. In intellectual and social life there are two forms of 

 energy, and it is the sum of these which determines the value of 

 the whole. With Helmholtz only a small part of the enormous 

 supply of energy stored up in him was actually in evidence at 

 any given moment. The conversion of his potential energy 

 into kinetic was slow, unlike what happens with those whom 

 people are wont to describe as geniuses. As he never worked 

 out the details of his lectures, but composed them as he went 

 along, he spoke slowly, deliberately, and at times a little 

 haltingly. His eyes looked away beyond his audience as 

 though he were seeking the solution of a problem at an infinite 

 distance. In the physiology classes he assumed no more know- 

 ledge and insight in his medical students than did any other 

 teacher of the same department. He seldom gave the names 

 of any experimenters, and least of all his own.' 



He was a keen teacher in the laboratory, and every earnest 

 student became one of his friends in science. As free from 

 professional jealousy as Magnus, whom he had so often com- 

 mended, he frequently supplied the fundamental ideas for the 

 splendid work that issued from his laboratory, and provided 

 a wealth of suggestions for the overcoming of new experimental 

 problems, in which more or less ingenuity was required. 

 1 Whoever had the luck/ says Bernstein, who for years was his 

 assistant at the Physiological Institute, 'to watch Helmholtz 



