322 PHYSICAL CHEMISTRY 



from complete, is affected by a considerable error. One example may 

 be cited : when thallous chloride and bromate, each of which alone 

 has a solubility of about ^ normal in water at 40, are simultaneously 

 present as solid phases, the solubility of each is reduced by the other 

 to an extent which shows that the concentration of the un-ionized 

 molecules is diminished by about 15 per cent and that the product 

 of the ion-concentrations is increased by about 5 per cent. This case 

 is a typical one; but what the quantitative law of the influence in 

 question is, can be determined only by a further study of the phe- 

 nomenon. In the case of tri-ionic salts, the ion-concentration product 

 is even approximately constant, only when the square not when 

 the first power of the concentration of the univalent ion is em- 

 ployed. This has been shown by experiments with lead iodide in the 

 presence of potassium iodide, with lead chloride in that of other chlo- 

 rides, and with calcium hydroxide in that of ammonium chloride. 



I will close by calling your attention to a remarkable principle in 

 regard to the properties of salt solutions, of a character quite distinct 

 from those thus far considered. That many properties of dilute salt 

 solutions can be expressed as the sum of values assigned once for all 

 to the constituent radicals or ions was long ago recognized, and has 

 often been cited as a corollary from the ionic theory. That this 

 additivity of properties persists up to fairly high concentrations is 

 a fact, however, that has received scant consideration, owing to its 

 apparent lack of relationship to that theory. This fact is shown strik- 

 ingly in the case of certain highly specific optical properties which are 

 ordinarily found to be dependent in a high degree on molecular struc- 

 ture. Thus, the experimental data fully warrant the statement of the 

 principle that the optical activity and the color of salts in solution, when 

 referred to equivalent quantities, are independent of the concentration and 

 therefore of the degree of ionization of the salts, and are additive with re- 

 spect to the properties of the constituent ions even up to concentrations 

 where a large proportion of the salt is in the un-ionized state. Abundant 

 data might be cited in support of this principle, especially with re- 

 ference to optical activity. But I can only illustrate the character 

 of the evidence by presenting a few of the results obtained by Wai- 

 den with the salts of a-brom-camphor-sulphonic acid. In -^ normal 

 solution he found the following values of the molal rotatory power: 



Lithium salt . . .275 Acid itself . - . . 273 



Sodium salt . . . 272 Beryllium salt . . . 274 



Potassium salt . . . 273 Zinc salt . . . 272 



Thallium salt . . .273 Barium salt . . .272 



The values are seen to be substantially identical, although the con- 

 ductivity shows the acid to have an un-ionized fraction of 7 per cent, 

 the salts of the univalent metals one of 16 per cent, and those of the 



