PRESIDENT S ADDRESS — SECTION A. 47 



Then von Nernst (/) came to the rescue. He propounded a theory, 

 "based on theresiiltsof osmotic-pressure experiments and Arrhenius' theory 

 of ionic dissociation, which, at first sight, looked brand new ; certainly 

 it was a daring, original, and highly successful attempt to express thie 

 •electromotive force of all known types of cell by means of a single 

 formula. The connection between Nernst's ideas and Helmholtz's 

 •calculations is not easily traced off hand, as their authors speak 

 ■different languages. The latter confines himself rigorously to mathe- 

 matical deductions from known facts, the former makes use of the 

 picturesque methods of molecular hypothesis. Nevertheless, it is 

 3)0ssible to strip Nernst's theory of its ionic dress — a process which 

 results in a decided increase in its generality — and when this is done 

 its connection with the older theory becomes apparent. What Nernst 

 xeally effected was, we find, the discovery of a general method of ex- 

 pressing the changes of free energ}' in terms of quantities ideally or 

 actually measurable. It is true that the method involves an element 

 •of hypothesis concerning which the last word has yet to be said, but it 

 is possible to make too much of this ; the use of the term " electrolytic 

 solution pressure " need involve only just as much — or should I say as 

 little ? — hypothesis as the ascription of specific inductive capacity to 

 a solid dielectric, each being, in the last resort, a matter of definition. 

 I admit that in Nernst's own way of introducing the expression the 

 hypothetical element is much larger, but that is owing to the manner 

 in which the theory originated ; it is of its accidents, not of its essence.(9) 

 liOoking at the matter from the purely practical standpoint, it is clear 

 that Nenist's own statement of his theory, like all working models of 

 natural processes, has a great advantage over abstract reasoning for 

 the average investigator. It has led to an immense amount of valuable 

 work at the hands of men who knew little more of thermodynamic 

 theory than they did of Accadian syntax. Even the man who prefers 

 to do his thinking in generalised co-ordinates frequently finds himself 

 none the worse off for the aid of a comfortable crutch of hypothesis ; 

 it is not, therefore, surprising that the results of the bulk of recent 

 work on electromotive forces should be stated in terms of ions and 

 osmotic partial pressures. 



During the last 20 years the old dispute between the contact and 

 ■chemical theories has quietly flickered out, mainly owing to the in- 

 creased precision which has gradually attached itself to the definition 

 of electromotive force. For this result Helmholtz is primarily re- 

 sponsible, though much is due to the clearness and force with which 

 Lodge {h) has followed up the lines of thought suggested by the doctrine 

 of energy : much also to Jahn (i) and Gill's (j) direct determination 

 of the points at which energy enters and leaves the circuit. Gait's (k) 

 determinations of the heat of formation of brass — to which Lord 



(/) Zeitsch. Phys. Chem., IV. 

 ig) I am not aware that this imderlyins; relationship between the theories of 



Helmholtz and Nernst has been indicated elsewhere. 



(k) B.A. Rep., 1885. 1887 ; Proc. Phys. Soc, Lond., XVII. ; Phil Mag. [5], XLIX. 



(i) Wied. Ann., XXXIV, L. (/) Wied. Ann., XL 



(^-)BIit. Ass. Rep., 1898-1899. 



