200 ESSAYS BIOGRAPHICAL AND CHEMICAL 



through the semi-permeable screen or partition, so the 

 electrons can pass backwards and forwards through the 

 metallic plates and wire; and just as the sugar molecules 

 are unable to traverse the membrane, so the matter with 

 which the electrons are in combination is unable to pass 

 through the metal. The metal is thus a semi-permeable 

 membrane, and electric pressure is developed in con- 

 sequence, in the same way as osmotic pressure is developed 

 by the sugar in solution. 



If a 'weak solution of common salt be boiled down, after 

 sufficient water has been evaporated away, crystals of salt 

 separate out and deposit. Now the weak solution con- 

 tains the constituents of the salt almost entirely in the 

 state of ions ; that is, the sodion is without an electron, 

 which, if added, would convert it into the metal sodium ; 

 and the chlorion would be the element chlorine, if it 

 could part with its electron. 



During concentration, as the water evaporates, the ions 

 of sodium and chlorine are brought nearer each other, 

 and they combine to form solid salt when enough water 

 has been removed. But even when combined to form 

 salt in the solid state, the electron does not leave the 

 chlorion and attach itself to the sodion ; if that happened 

 the result would be metallic sodium and chlorine gas; 

 and they are certainly not formed. A crystal of salt 

 differs from a solution of salt in much the same respects 

 as a piece of ice differs from water ; the one is solid and 

 the other is liquid ; but evidently the same stuff is there ; 

 the only difference is in the solidification. 



It must therefore be supposed as a legitimate inference 

 that when a lump of sodium unites with chlorine and 

 burns in it as a lump of coal burns in air, the act of 

 combination consists of the transference of an electron 

 from the sodium metal to the chlorine ; the result of this 

 transference is to convert the sodium metal into sodions 



