Pharmacodynamics of Salts and Drugs 93 



A word may be said about the reliability of these figures. They 

 depend, as will be seen, upon direct measurements of the electromo- 

 tive force shown between two metals when immersed in known solu- 

 tions of their salts, those solutions being in contact. This assumes a 

 knowledge of the number of ions of metal in the salt solutions in ques- 

 tion, and this factor is not in all cases perfectly certain. A much more 

 serious source of possible error arises from a determination of the 

 potential of any single metal, since it is necessary to know the poten- 

 tial of at least one electrode before the rest can be determined. The 

 measurement is ordinarily made by using the calomel mercury elec- 

 trode, which is supposed to have a potential of +0.56 volts. This 

 voltage was determined by measuring the potential between a drop- 

 ping mercury electrode, which theoretically should have a zero poten- 

 tial, and the calomel mercury electrode. Recent determinations of 

 the potential of the calomel mercury electrode by a totally diflferent 

 method by Billitzer' give a different value. It is, therefore, impos- 

 sible at present to say which of these measurements or methods gives 

 the more reliable result. Nernst has accordingly proposed that the 

 hydrogen electrode in normal ionic solution be regarded arbitrarily 

 as zero, until the absolute potential is determined. I have, however, 

 used the values as given by Wilsmore, based on the calomel electrode 

 0.56. 



The question does not influence most of the measurements which 

 follow, since these are based on the sum of the potentials of both 

 anion and cation, or upon the differences between two like ions. 

 This is a constant whatever the absolute potential, since if a certain 

 number is added to the anion, it will be subtracted from the cation. 

 For example, suppose it be shown by a change in the point of zero 

 potential that the solution tension of sodium is 2 . 34 instead of 2 . 54, 

 this increases the ionic potential by 0.2 volt; but if sodium is 2.34, 

 then chlorine is i . 894 instead of i . 694, and this reduces the ionic 

 potential of chlorine by 0.2 volt. The sum of the potentials of 

 sodium and chlorine remain unchanged. 



b) The relation between physiological action and ionic potential. — 

 To bring out the relationship between ionic potential and toxic action, 

 I have prepared Table 3, which shows this relationship for the toxic 



■ BiLtrrzER, Ztschr. fur Eiektrochemie, 1902, 8, p. 638. 



