A HUNDRED YEARS OF CHEMISTRY. 65 



pressure, temperature, and volume among gases find their equiva- 

 lents in the phenomena exhibited by solutions. In Avogadro's 

 law we learn that equal volumes of gases, under like conditions of 

 temperature and pressure, contain equal numbers of molecules. 

 According to the new generalizations, equal volumes of different 

 solutions, if they exert the same osmotic pressure, also contain equal 

 numbers of molecules. The parallelism is perfect. With these 

 relations the freezing- and boiling-point phenomena are directly 

 connected. 



But, both for gases and for solutions some apparent anomalies 

 existed. Certain compounds, when vaporized, seemed not to con- 

 form to Avogadro's law, and called for explanation. This proved 

 to be simple, and was supplied by the fact that the anomalous com- 

 pound, as such, did not exist as vapor, but was split up, dissociated, 

 into other things. For instance, ammonium chloride, above a cer- 

 tain temperature, is decomposed into a mixture of two gases — hy- 

 drochloric acid and ammonia — which, on cooling, reunite and repro- 

 duce the original compound. Twice as much vapor as is required 

 by theory, and specifically half as heavy, is produced by this trans- 

 formation, which is only one of a large class, all well understood. 



In the case of solutions it was found that certain compounds, 

 notably the acids, alkalies, and metallic salts, caused a depression 

 of freezing point which was twice as great as ought to be expected. 

 This fact was illuminated by the phenomena observed in gases, 

 and soon it was seen that here too a splitting up of molecules, a 

 true dissociation, occurred. These anomalous solutions, moreover, 

 were electrolytes — that is, they conducted electricity and under- 

 went electrolytic decompositions — while normal substances, espe- 

 cially solutions of carbon compounds, such as sugar, were not. 



Van t'Hoff's discoveries went far, but one more step was needed, 

 and this was taken by Arrhenius in 1888. Electrolytic compounds, 

 when dissolved, are actually dissociated into their ions, partially so 

 in a strong solution, entirely so in one which is infinitely dilute, 

 a statement which leads to some extraordinary conclusions. For 

 instance, the ions of common salt are sodium and chlorine. In a 

 dilute solution the salt itself ceases to exist, while atoms of sodium 

 and atoms of chlorine wander about, chemically separated from 

 each other but still in equilibrium. Sodium sulphate may be re- 

 garded as made up of two parts — sodium and an acid radicle which 

 contains one atom of sulphur to four of oxygen — and these parts, 

 its ions, are severed apart during solution to move about independ- 

 ent of each other. 



This theory of Arrhenius, the theory of electrolytic dissociation, 

 is supported by many facts, and fits in well with the kinetic theory 



