PEOGEESS IN CHEMISTRY. 253 



Stockholm, hiid .shown that the conductivit}' of salt .solutions for elec- 

 tricity may be explained on the assumption that when a .salt, such as 

 KNO3, is di.s.solved in w^ater it di.s.sociates into portions similar in num- 

 ber and kind to those it would yield if electrolyzed and if no second- 

 ary reactions were to take place. Such portions (K and NO^ for 

 example) had been named ions l)y Faraday. The conductivit}^ of such 

 solutions l)ecomes greater, per unit of dissolved salt, the weaker the 

 .solution until finally a limit is reached, after which further dilution no 

 loiio-ci- increases conductivit}'. Now Van't Hoff united all these iso- 

 lated observations and showed their hearing on each other. Stated 

 shortly the hypothesis is as follows: When a substance is dissolved in 

 a large quantity of a solvent its molecules are separated from each 

 other to a distance comparable with that which obtains in gases. They 

 are, therefore, capable of independent action and when placed in a 

 vessel the walls of which are permeable to the solvent, l)ut not to the 

 dissolved substance ('*.semipermea])le membrane"), the imprisoned 

 molecules of the latter exert pressure on the interior surface of these 

 walls as if they were gaseous. Van't Hoti" showed the intimate con- 

 nection between this phenomenon and the depression of freezing point 

 and vapor pressure already alluded to. He pointed out further that 

 the exceptions to this behavior, noticed in the case of dissolved .salts, 

 are due to their "electric dis.sociation " or "ionization," as it is now 

 termed; and that in a sufficiently dilute solution of potassium nitrate, 

 for example, the osmotic pressure and the correlated depression of 

 freezing point and rise of boiling point are practically equal to what 

 would be produced were the salt to be .split into its ions, K and NO3. 

 These views were vigorously advocated by Ostwald (professor at Leip- 

 zig) in his " Zeitschrif t f iir physikalische Chemie," and he and his pupils 

 have done nmch to gather together facts in confirmation of this theory 

 and in extending its scope. 



It must be understood that the ions K and NO3 are not, strictly 

 speaking, atoms; they are charged atoms; the K retains a + and the 

 NO3 a — charge. On immersing into the solution the poles of a bat- 

 tery, one charged + and the other — , the +K atoms are attracted 

 to the — pole and are there discharged; as soon as they lose their 

 charge they are free to act on the w^ater, when they liberate their 

 equivalent of hydrogen. Similarly, the -NO3 groups are discharged 

 at the -f pole and abstract hydrogen from the water, liberating an 

 equivalent quantity of oxygen. Thus the phenomenon of electrolysis, 

 so long a mysterious process, finds a simple explanation. The course 

 of ordinary chemical reactions is also readily realized when viewed in 

 the light of this theory. Take, for example, the ordinary equation: 



AgN0,.Aq+NaCl.Ag=AgCl+NaN03.Aq; 

 i. e., solutions of silver nitrate and sodium chloride give a precipitate 



