June 6, 1902.] 



SCIENCE. 



893 



ions is always very much greater. Indeed 

 we may quite correctly say that in most 

 highly dissociated solution of hydrochloric 

 acid the hydrogen and the chlorine are still 

 very firmly united, not indeed atom to atom, 

 but each atom of the one kind to all the 

 atoms of the other kind within a certain 

 distance from it. A man does not lose 

 his money when he takes it out of his pocket 

 a.nd puts it into a bank. He does indeed 

 lose his relation to the individual gold and 

 silver coins, and does not know and does 

 not care where these particular pieces of 

 metal are, but he is interested in knowing 

 that they or their like are at his command, 

 and the same sort of work will be required 

 to impoverish him whether his money is 

 in the bank or in his pocket. (I assume, 

 of course, that the bank of our present im- 

 ' agination cannot become insolvent.) 



I have said that the test of the theory 

 would come later. It has been going on 

 since 1887, and if time would allow I could 

 give you many cases in which deductions 

 from the theory have been found to agree 

 with close quantitative accuracy with ex- 

 perimental observations. I shall mention 

 only the first, still among the most impor- 

 tant, namely, Ostwald's determination of 

 the affinity constants, and his application 

 -of Guldberg and Waage's principle to the 

 ions. I could also give you instances in 

 which there have been discrepancies, or 

 apparent discrepancies, and show how in 

 some of these cases the difficulties have been 

 cleared up. The history of this theory has 

 in fact so far been that of every useful 

 theory, for it is in this way only that a 

 theory does its work. I shall select two 

 points for illustration, not because they 

 are more important than others, but be- 

 cause I can illustrate them by means of 

 experiments which do not occupy much 

 time, and can be made visible in a large 

 room. The first has reference to the ques- 

 tion, What are the ions in the case of a dibasic 



acid? As HNOg gives as its ions H and 

 NO3 so we might expect H2SO4 to give 2H 

 and SO4. But we find that until the dilu- 

 tion has advanced to a considerable extent 

 the ions of sulphuric acid are mainly H 

 and HSO4. This is quite in harmony with 

 the chemical action of II2SO4, for, as every 

 chemist knows, at moderate temperatures 

 we have the action H2S04 + NaCl=HCl 

 +NaIIS04, and the temperature has to be 

 raised in order to get the action NaHSO^ 

 -fNaCl^HCl-fNa^SO,. In the first of 

 these experiments we take as the electrolyte 

 a concentrated solution of potassiiun hydro- 

 gen sulphate KHSO^. This gives the ions 

 K and HSO4. The latter go to the anode 

 and there, on being discharged, form per- 

 sulphuric acid, or its ions, and potassium 

 persulphate KoSjOs, being sparingly solu- 

 ble, crystallizes out. This is the method 

 by means of which Dr. Marshall discovered 

 the persulphates. The next experiment 

 will illustrate the formation and discharge 

 at the anode of the anion SO4. We have 

 here dilute sulphuric acid with which is 

 mixed a little manganous sulphate MnSOi- 

 The ion SO4 when discharged, adds itself 

 to 2MnS04 and forms manganic sulphate 

 MnjC 804)3, recognized by its red color. 

 This, even in acid solution, is quickly 

 hydrolyzed, giving insoluble manganic 

 hydrate. 



The other point I wish to illustrate is the 

 application of Guldberg and Waage 's prin- 

 ciple to ions. Without entering into any 

 general discussion of this question, I shall 

 merely say that theory leads to the result 

 that the addition of a soluble acetate to a 

 solution of acetic acid diminishes the con- 

 centration of H ions, and so makes the solu- 

 tion less effectively acid. This was experi- 

 mentally proved by Arrhenius in 1890,* 

 by measuring the rate at which cane-sugar 



* Arrhenius, Zeitschrift f. physik. Chemie, V., 

 p. 1 (1890). 



