May 4, 1921 wherry and adams: acidity 199 



ion concentration of pure water is not exactly known, and that it 

 varies with the temperature, need introduce no difficulty, just as in 

 our measuring of lengths it is not a vital matter that the meter is 

 not an exact fraction of the earth's circumference. 



(2) The Xh values are not, while the Ph values are, "directly de- 

 rived from the potential of the normal hydrogen electrode." 



(Ans.) Measurements are actually made against a half-cell, such 

 as the calomel electrode, whose potential in the scale used, whether 

 Ph or Xh, must be independently determined in any case. The factor 

 by which the results of measurement must be multiplied is the same 

 in both. 



(3) Factory workmen can gain a correct picture of the relation of 

 Ph values to a given process and are not bothered by the fact that the 

 scale runs in the reverse direction from those in everyday use. 



(Ans.) People of even greater intelligence than the average workman 

 can be taught to use the Baumc hydrometer scale, the Wedgwood 

 pottery-temperature scale, and other empirical atrocities. 



(j/) "Quantities of important data are already recorded in terms 



of pH " "To have two scales with numerical values so similar 



might produce a confusion " 



(Ans.) If this sort of consideration had been taken seriously in the 

 past, we should still be using Siemens mercury units of electrical re- 

 sistance instead of ohms; Dalton's instead of Berzelius's symbols for 

 the chemical elements; and element symbols which would represent 

 the jumble of equivalent and atomic proportions current before the 

 time of Cannizzaro. 



Finally, this fact may be emphasized: Only if the validity of the 

 laws of dilute solutions be assumed, is the potential due to hydrogen 

 ion proportional to the logarithm of the concentration of that ion, 

 that is, the Ph value; and yet the Ph scale takes its origin in the very 

 region (normality) in which these laws do not hold. 



Is it, then, worth while to continue the use of the Ph system? 



PHYSICAL CUUMISTRY.— Reply to Wherry and Adams' article 

 on methods oj stating acidity.'^ Wm, Mansfiei^d Ci.ark, Hy- 

 gienic Laboratory, 



Wherry and Adams, in their reply to my criticism of xH, have failed 



^ Received February 2, 1921. The notation used in Dr. Clark's book is followed in 

 this article. — The Editors. 



