MIRACLES OF SCIENCE 



are the^precise relations that obtain between 

 the particles of the salt and the particles of the sol- 

 vent. The best authenticated theory as to what takes 

 place is that which Svante Arrhenius, the famous 

 Swedish chemist, put forward as long ago as 1887, 

 and which has been more or less matter of contro- 

 versy ever since. 



In accordance with this theory, which is known 

 as the ion theory, a portion at least of the molecules 

 of salt are broken up in the process of solution into 

 so-called ions, each or which consists of an atom 

 charged with electricity. A molecule of salt as is 

 well known, is composed of one atom of sodium and 

 one atom of chlorine. In the salt solution, according 

 to the theory of Arrhenius, each liberated atom of 

 sodium would convey a unit charge of positive elec- 

 tricity and each liberated atom of chlorine a unit 

 charge of negative electricity. 



The ions would be free to move about in the 

 solvent, and their capacity for such motion is demon- 

 strated when an electric current is passed through 

 the solution. In such an event, the positively charged 

 ion is moved toward the negative pole, and the nega- 

 tively charged ones toward the positive pole. The 

 fact of such migration is demonstrated, in the case 

 of solutions of metallic salts, in the familiar process 

 of electro-plating, in which, as is familiarly known, 

 particles of pure metal, copper or silver, are deposited 

 at the negative electrode. It is believed that the ion 

 in its migration carries with it a film of the solvent. 

 The rate of migration was first measured by Sir 

 Oliver Lodge, and various investigators have re- 



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