394 POPULAR SCIENCE MONTHLY. 



truni for the second. A similar observation may be made for the 

 tetraiodine-fluoresceine and its potassium salt. In general the figure 

 shows that the spectrum is changed in a very conspicuous manner at 

 the smallest chemical change of the molecule. 



It might therefore be expected, after the old manner of view, that 

 the replacement of hydrogen by a metal in permanganic acid, or of 

 one acid rest by another in the salts of para-rosaniline, would wholly 

 change the character of the spectra. This is not the case, as Ostwald 

 has shown. The spectra are wholly unchanged, as Figs. 8 and 9 show. 

 The spectra are all produced by the same substance, viz., the perman- 

 ganate-ion, in the one, the para-rosaniline-ion, in the other case. Only 

 in the case of the para-rosaniline salts we observe that the absorption 

 is sensibly weaker in some cases than in others. The weakening de- 

 pends upon the hydrolysis of the salts of the weak acids, e. g., acetic 

 and benzoic acids. This research of Ostwald shows in a most con- 

 vincing manner the correctness of the views of the theory of electro- 

 lytic dissociation. 



It has been objected to this theory, that according to it it might be 

 possible by diffusion to separate both ions, e. g., chlorine and sodium, 

 from another in a solution of sodium chloride. In reality chlorine 

 diffuses about 1.4 times more rapidly than sodium. But the ions carry 

 their electric charges with them. Therefore if we place a solution of 

 sodium chloride in a vessel and we pour a layer of pure water over it, 

 it is true that in the first moments a little excess of chlorine enters the 

 water. By this the water is charged negatively, and the solution under 

 it positively, so that the sodium ions are driven out from the solution 

 with a greater force than the chlorine ions. As soon as that force is 

 1.4 times greater than this, the chlorine ions travel just as slowly as 

 the sodium ions. It is not difficult to calculate that this case happens 

 as soon as the chlorine ion is contained in the water in an excess of 

 about the billionth part of a milligram over the equivalent quantity of 

 sodium. This extremely minute quantity we should in vain try to 

 detect by chemical means. By electrical means it succeeds pretty well, 

 as Nernst has demonstrated experimentally for his concentration ele- 

 ments. Therefore, the said objection is valid against the hypothesis 

 of a common dissociation of the salts, but not against a dissociation 

 into ions, that are charged with electricity, as Faraday's law demands. 

 Probably this objection has hindered an earlier acceptance of a disso- 

 ciated state of the electrolytes, to which, for instance, Valson and 

 Bartoli inclined. 



The gaseous laws that are valid for dilute solutions have made the 

 calculation of the degree of dissociation possible in a great number of 

 cases. The first application of that nature was made by Ostwald, who 

 showed that the dissociation equilibrium between the ions and the non- 



