382 SCIENCE PROGRESS 



the molecular conductivity at this dilution with the limiting 

 molecular conductivity when ionisation is complete. 



Not long before, Van 't Hoff (1885) had brought forward his 

 theory of solution, according to which a substance in dilute 

 solution obeys the gas laws, osmotic pressure being taken as the 

 analogue of gas pressure, and, further, the osmotic pressure of 

 a dissolved substance is numerically equal to the ordinary 

 pressure which it would exert if present in the gaseous form in 

 the same volume as is occupied by the solution. Van 't Hoff 

 found, however, that the osmotic pressure of salts, strong acids, 

 and bases in aqueous solution (obtained indirectly from Raoult's 

 freezing-point and boiling-point determinations) was greater 

 than the calculated value — the solutions behaved as if more 

 particles were present than were to be expected according to 

 the amount of substance used — and it was found necessary to 

 introduce a factor i, which represented the ratio of the observed 

 and the calculated osmotic pressures. 



The chemical world had not long to wait for an ingenious 

 and fruitful hypothesis as to the significance of Van 't Hoff's 

 factor i. Arrhenius suggested that the apparent increase in the 

 number of particles was due to ionisation, and, in his classical 

 paper in the Zeitschrift fur physikalische Chemte, 1 gives a table 

 in which the coefficient i is calculated from the results of 

 conductivity and of osmotic pressure measurements for over 

 ninety substances, including non-conductors, acids, bases, and 

 salts. The agreement is in the great majority of cases extremely 

 satisfactory. 



It would seem to be a severe test of the theory to apply it 

 to cases where the possible number of ions derived from a single 

 molecule is large. Thus the normal sodium salt of hexabasic 

 mellitic acid, C 6 (COONa) 6 , should dissociate according to the 

 equation, C 6 (COONa) 6 = 6Na + C u (COO) 6 """, and may therefore 

 be expected to have an osmotic pressure in dilute solution 

 about seven times as great as that of a solution of cane sugar 

 of equivalent concentration. It has been shown indirectly, by 

 freezing-point determinations, that this value is nearly reached 

 in o - ooi8 molar solution, so that the requirements of the theory 

 are satisfactorily fulfilled. 



The introduction of the electrolytic dissociation theory has 

 been abundantly justified by the great number of diverse facts 



1 Zcit. physik. chcm. 1887, i. 631. 



