Wells & Foote — One Hundred Years of Chemistry. 289 



the same volume. Further investigations have fully 

 established the fact that molecules in dilute solution obey 

 the simple laws of gases. 



It was pointed out by van't Hoff* that salts, strong- 

 acids and strong bases showed marked exceptions to his 

 law in exerting much greater osmotic pressures than 

 those calculated for them. 



The next year in 1887, Arrhenius explained this abnor- 

 mal behavior of salts, strong acids and strong bases by 

 assuming that they dissociate spontaneously into ions 

 when they dissolve, and that these more numerous par- 

 ticles act like molecules in producing osmotic pressure. 

 He showed that these exceptional substances all conduct 

 electricity in solution, while those conforming with van't 

 HofT's law do not, and according to his theory the ions 

 become positively or negatively charged when they are 

 formed, and these charged ions conduct the current. 

 For example a molecule of sodium chloride was supposed 

 to give the two ions Na+ and Cl — , thus exerting twice as 

 much osmotic pressure as a single molecule. 



Determinations of osmotic pressure or related values, 

 such as depression of the freezing point and of electric 

 conductivity, indicated that ionization could not be 

 regarded as complete in any case except in exceedingly 

 dilute solutions, and that the extent of ionization varied 

 with different substances. The fact that osmotic pres- 

 sures and electric conductivities gave closely agreeing 

 results in regard to the extent of ionization in various 

 cases, is the strongest evidence in support of the theory. 



It was difficult at first for many chemists to believe 

 that atoms, such as those of sodium and chlorine, and 

 groups such as NH 4 and S0 4 could exist independently 

 in solution, even though electrically charged. However, 

 the theory rapidly gained ground and is now accepted 

 by nearly every chemist as a satisfactory explanation of 

 many facts. 



During recent years, many investigations relating to 

 osmotic- pressure and ionization have been carried out in 

 the United States, but only the work of Morse, A. A. 

 Noyes, and the late H. C. Jones can be merely alluded to 

 here. It should be mentioned that the eminent author 

 of the ionic hypothesis gave the Silliman Memorial course 

 of lectures at Yale in 1911 on Theories of Solution. 



Am. Jour. Sci.— Fourth Series, Vol. XLVI, No. 271.— July, 1918. 

 10 



