352 



EEPOET— 1886. 



' Chloride of potassium gave in one experiment- 



' Anhydrous salts behave in general like KCl, that is to say, the ratio X is very 

 near 2, even for moderate concentrations. Hydrated salta, on the other hand, are 

 comparable to sulphate of zinc, and it is only for very gi-eat dilutions that their 

 ratio X approaches the value 2.' 



M. Boutv considers that at sufficient dilution the molecular conductivity of all 

 salts is the same. For salts without water of crystallisation a concentration of 

 -i- or so may be permitted ; but for hydrated salts it is necessary to go below 

 these limits, a thing which requires the possession of absoluteh/ pure xoater, and he is 

 only able to show that they approach equality to the other class of salts when very 

 attenuated. Two tables follow, showingratios of resistances compared with ratios of 

 concentration, first for anhydrous salts, and then for salts which crystallise with a 

 definite amount of water, KCl being taken as the standard of reference. 



IV. On the Migration of Ions, and its Relation ivith the Conductivity of 



Salt SoluHons. 



The author styles the case when the migration number of each ion is simply ^ 

 as 'normal electrolysis,' and he quotes results of Hittorf to show that the salts 

 which thus behave are anhydrous salts, e.g., AmCl, KCy, KCl, K.,SO^, K^CrO^, 

 KNO3, KBr, KCIO3, KCIO,, Ag,SO„ KI,_AgN03. 



For all these Hittorfs number {n) is very near -5 and scarcely varies with 

 dilution. But for salts which definitely combine with water neither of these 

 statements holds. Here are Hittorfs numbers. S represents the weight of water 

 combined with 1 gramme of the salt ; n expresses the loss of concentration at the 

 cathode while 1 equivalent of electrolyte is decomposed. 



It seems from the following table that hydrated salts approach normality as 

 their solutions become very dilute. They then also tend to obey the law of 

 equivalents (kjin = const.) ; so the two things are connected. 



' When a salt obeys the law of equivalents, its electrolysis is normal (migration 

 number 5) ; wheu it does not obey, it is abnormal, and the divergence which the law 

 presents to the law of equivalents is so much the greater as the number n differs 

 more from the normal value -5. . . . There is thus, between the resistance of 

 salt solutions and the phenomenon of migration, a very close relation impossible to 

 overlook.' 



[' This ' ratio A.' is plainly the same thing as Arrhenius' ' exponent of dilution,' 

 only more simply introduced. I have shown in remarks on Arrhenius' memoir (first 

 part, see below, p. 360) that the meaning of this ' ratio X,' in so far as it is con- 

 stant, is that conductivity varies as a power of the concentration, h oc w'', where 

 r = log A./log2.] 



i 



