14 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Hydrochloric acid evidently occupies a position quite anomalous, and 

 ammouie chloride also, at the other extreme, produces an electrode with 

 a temperature coefficient considerably different from the average of the 

 other metals. The presence of hydrogen ions in the one case, and the 

 tendency toward the formation of amido compounds in the other might 

 well tend to separate these two cases from the others, hence the consider- 

 ation of these substances will be postponed. The remarks which imme- 

 diately follow apply to the other chlorides. 



It is interesting to note that each successive dilution produces about 

 the same change in the temperature coefficient (on the average 0.00019 to 

 0.00018 volt). In other words, if we take the concentration of the 

 centinormal solution as 1, of the decinormal solution as 10, and of the 

 normal solution as 100, it is manifest that the changes in the tempera- 

 ture coefficient are proportional, not to the changes in the concentration 

 itself, but to the changes in the logarithm of the concentration. 



This relation immediately reminds one of the Nernst formula, in 

 which the potential itself is a logarithmic function of the osmotic pres- 

 sure of the ions; for this osmotic pressure is with dilute solutions nearly 

 proportional to the concentration of the salt in the solution. With this 

 idea in view, it becomes at once a matter of interest to correct the results 

 given above for the concentrations of the respective ions ; it is obvious 

 that the normal solution of cadraic chloride must be much weaker in 

 chlorine ions than a normal solution of potassic chloride. In order to 

 attain this end one must first construct a logarithmic formula for the 

 temperature coefficient involving the concentration of the surrounding 

 electrolyte ; and naturally the Nernst formula is the most convenient 

 starting point for this construction. Considering the electrode in question 

 in the simplest way as a source of negative ions, its potential may be 

 assumed to have this value : — 



RT , P 



7r ^ 



e, 



hi — , 



P 



in which tt = the potential difference in question, R= the gas-constant 

 = 1.96 X 4,24 volt-coulombs, eo = 96,540 coulombs. In indicates a nat- 

 ural logarithm, P is the " outward tendency " (solution tension) of the 

 chlorine in the electrode, p is the osmotic pressure of the chlorine ions 

 in the surrounding solution, and T is the absolute temperature. 

 The differential of this equation is 



a-n- = — 



Co 



V \ P p / 



p \ I^ p / A 



