384 HERMANN VON HELMHOLTZ 



certain and established, upon the so-called Second Law of 

 Thermodynamics, which (at first stated by Sadi Carnot in a 

 restricted form that applied only within the narrowest limits 

 of temperature) had been extended and generalized in the strict 

 interpretation given to it by Clausius. ' This is not merely one 

 of the most important, but also one of the most surprising and 

 original achievements of ancient or modern physics/ because it 

 is one of the very few principles whose absolute universality 

 can be predicated independent of all dissimilarity of natural 

 bodies. At the close of his laudatory appreciation of the work 

 of Clausius, Helmholtz states that it had now for the first time 

 become possible to obtain a concept of absolute temperature, 

 independent of the characteristics of any particular natural 

 body. Still more important was the fact that it had established 

 the specific character of the motions of heat, by which the latter 

 were differentiated from all other force-equivalents. While the 

 others can be converted and reconverted indefinitely among 

 themselves, this is only possible to a very limited degree for 

 heat, so long at any rate as we are unable to go back to the zero 

 of absolute temperature. 



What Helmholtz himself thought in regard to these difficult 

 questions appears from the preceding account of his mono- 

 cyclic studies, in which he distinguishes between organized 

 motion, defined as a continuous function of the co-ordinates and 

 the time, and unorganized motion, in which the motions of the 

 neighbouring particles have no kind of mutual similarity. He 

 regards the motion of heat as being also unorganized, but 

 refers the difficulty of converting it into organized motion 

 solely to the limitations of the methods at our disposal ; if we 

 could but overcome this obstacle, all processes would necessarily 

 be reversible. Moreover, as he repeatedly indicates, there are 

 vegetative processes in many plants, where no source of energy 

 is visible, and the question presents itself whether these may 

 not in some sort be the organization of heat motion. 



During the Easter holidays Helmholtz heard of the death of 

 his faithful old friend Bonders, and writes on March 27 to 

 Engelmann : ' Your announcement of the death of Professor 

 Bonders was a quite unexpected blow, and a great shock to me. 

 I knew no other scholar and distinguished investigator in whom 

 the consciousness of working for ideal aims was so keen and so 



