64. POPULAR SCIENCE MONTHLY. 



ing numbers. One equivalent weight of zinc consumed in the gal- 

 vanic battery yields a current which will deposit one equivalent of 

 silver from its solution, or which, decomposing water, will liberate 

 one equivalent each of oxygen and hydrogen. All electro-chemical 

 changes followed this simple law, which gave new emphasis to the 

 atomic theory, and furnished a new means for measuring the com- 

 bining numbers. 



In the early days of electro-chemistry the products of electroly- 

 sis were studied in the light of the dualistic theory. But as chem- 

 ical investigation along other lines overthrew this hypothesis, a 

 closer examination of electrolytic reactions became necessary. 

 Electrical decompositions were dualistic in character, but the dual- 

 ism was not that taught by Berzelius. When a salt, dissolved in 

 water, is decomposed by the current it is separated into two parts, 

 which Faraday called its ions; in Berzelian terms these were in 

 most cases oxides, but this conclusion fitted only a part of the facts, 

 and finally was abandoned. Whatever the ions might be, they 

 were not ordinary oxides. 



Many and long were the investigations bearing upon this sub- 

 ject before a satisfactory settlement was reached. The phenom- 

 ena observed in solutions, raised still another question, that of the 

 nature of solution itself, and this is not yet fully answered. Two 

 lines of study, however, have converged, within recent years, to 

 some remarkable conclusions, the latest large development of chem- 

 ical theory. 



It has long been known that solutions of salts do not freeze 

 so easily as pure water, and also that their boiling points are higher. 

 In 1883 Raoult discovered a remarkable relation between the freez- 

 ing point of a solution and the molecular weight of the substance 

 dissolved, a relation which has since been elaborately studied by 

 many investigators. From either the freezing-point depression or 

 the elevation of the boiling point the molecular weight of a soluble 

 compound can now be calculated, and many uncertain molecular 

 weights have thus been determined. 



Another phenomenon connected with solutions, which has re- 

 ceived much attention, is that of osmotic pressure. A salt in solu- 

 tion exercises a definite pressure, quantitatively measurable, which 

 is curiously analogous to the pressure exerted by gases. In a gas 

 the molecules are widely separated, and move about with much 

 freedom. In a very dilute solution the molecules of a salt are 

 similarly separated, and are also comparatively free to move. The 

 kinetic theory of gases, therefore, is now paralleled by a kinetic 

 theory of solutions, founded by Van t'lloff in 1887, which is 

 now generally accepted. All the well-established laws connecting 



