THE GROWTH OF CHEMICAL IDEAS 133 



his work, the physical chemists developed another great chem- 

 ical idea, the theory of electrolytic dissociation, first advanced 

 by the S\vedish chemist Svante Arrhenius. Arrhenius' theory 

 arose from the application of the gas laws to chemical solu- 

 tions by the Dutch chemist van't Hoff. Just as the pressure 

 of a gas is a measure of the concentration of the gas molecules, 

 so the osmotic pressure of a solution, which is the pressure 

 produced through a semi-permeable membrane that transmits 

 the solvent but not the molecules of the material dissolved, 

 is a measure of the concentration and, thus, of the molecular 

 weight of the substances present. In dilute solutions of salts 

 this principle, which held beautifully for solutions of sugar, 

 failed until Arrhenius introduced the conception that salts 

 in solution dissociated into unit particles that were oppositely 

 charged electrically. Faraday had already postulated such 

 charged particles to explain the conduction of an electric 

 current through a solution and had termed them ions. 



It is now recognized that the simple picture developed by 

 Arrhenius is not adequate to account quantitatively for the 

 behavior of solutions of electrolytes, although his funda- 

 mental concept of dissociation is still the basis of the modern 

 theories of Peter Debye, E. Huckel, J. N. Bronsted, and 

 others. Today we do not consider the behavior of the single 

 ion, but the potential forces of the whole system of ions, in 

 which each is acted upon by the electrostatic field created by 

 the others. From such considerations, we can calculate with 

 reasonable accuracy many of the thermodynamic properties 

 of solutions, and can predict something of salt and ion ef- 

 fects as related to rates of reactions. 



As the chemical elements were identified and their atomic 

 weights were determined, it became possible to discern a sort 

 of order in their properties. They could be classified into 

 families whose chemical properties were similar. Thus, there 

 are the alkali metals, the alkaline earths, the halogens, and 

 so on. The compounds of sulfur resemble those of oxygen 

 far more closely than they do those of nitrogen, which, how- 

 ever, are akin to those of phosphorus. As a result of similar 



