Atoms, Ions and Molecules. 409 



such a colored solution may be evaporated to dryness and thor- 

 oughly desiccated at 100° without losing anything of the 

 intensity of its color. That is, the color persists under condi- 

 tions in which dissociation is impossible. 



2d. It makes the color or absence of color, as the case may 

 be, of the solution of the acid indicator when isolated depend 

 upon the absence of dissociation. But it is a familiar fact that 

 by sufficient dilution even the weakest acids are largely or even 

 completely dissociated.* 



Let us take litmus for example. This is a weak acid red dye- 

 stuff which in contact with an alkali becomes blue. According 

 to the explanation here criticized, isolated red litmus has a blue 

 ion which does not appear in solution because not dissociated. 

 But it is certain that with sufficient dilution it must be disso- 

 ciated, and according to this explanation it should then turn 

 blue. Whereas red litmus remains red with any amount of dilu- 

 tion so loug as any trace of color is left. 



Again it is said that blue litmus owes its blue color to disso- 

 ciation. But the blue solution may be evaporated and desic- 

 cated at 100° without losing its blue color. The color is, 

 therefore, not due to dissociation. 



With these complex and faintly acid organic substances it 

 seems probable that some change in their constitution takes 

 place in the presence of an alkali, such that the anion that 

 separates from the dilute solution of the isolated substance is 

 not the same as that which separates from the alkaline combi- 

 nation. That is to say, the anion which separates from red lit- 

 mus is not absolutely the same as that which separates from 

 blue. Only in this way can the various reactions of acid indi- 

 cators be satisfactorily explained. The reactions of these sub- 

 stances have not been studied with sufficient accuracy to indi- 

 cate exactly what is the change that takes place in the presence 

 of an alkali. It may be either a re-arrangement of some of 

 the atoms or it may depend on the taking up of one or more 

 molecules of water. 



When an alcoholic solution of phenolphthaleine is dropped 

 into much water, the resulting solution cannot be distinguished 

 from pure water by its color. The addition of a trace of 

 potash brings it to a deep rose red. Phenolphthaleine acts as 

 a very weak acid ; its very dilute solution must, therefore, be 

 dissociated, and the colorless anion of the isolated substance 

 must differ from the colored anion which separates from the 

 potash compound. Also, a solution of the potash compound so 

 dilute as to leave a transparent him, has been evaporated and 

 then desiccated for many hours at 100°, the color of the solu- 



* All their molecules become active, Ostwald, Lehrb., 2d G-. ed., ii, 653 ; Nernst, 

 Theoret. Ch., 440. 



