176 CHEMICAL DISCOVERY AND INVENTION 



32, the salt is wholly in the hydrated state, and that above that 

 temperature it is wholly anhydrous. It is pretty certain that 

 part of the salt is in both conditions throughout the whole range 

 of temperature. Both the two compounds just referred to are 

 salts, and as already mentioned such substances differ from 

 sugar in the fact that when an electric current is passed through 

 them in a state of solution, or when melted they undergo a pecu- 

 liar decomposition called " electrolysis." 



This subject is so important and has of late years occupied so 

 much of the attention of chemists that it deserves a somewhat 

 close consideration. 



Electricity has long been known to be capable of passing 

 through matter in two ways. Metals, and some other bodies in a 

 less degree, allow a current to pass through them with com- 

 paratively little loss. They are called " conductors," and silver 

 and copper are among the best conductors, while platinum, tin, 

 and lead are less good. When a metal like platinum or a non- 

 metal such as carbon, as in an Edi-swan incandescent lamp, is 

 used to carry a current of electricity a part of the energy dis- 

 appears as electricity, and appears as heat, but no chemical 

 change is produced. On the other hand, if a current is passed 

 through a solution of an acid, a base, or a salt the compound is 

 resolved into two parts which are liberated at the opposite poles 

 or electrodes. Such substances are called " electrolytes." Sugar, 

 alcohol, and neutral substances generally, such as chloroform, 

 ether, etc., are not electrolytes, and offer great resistance to the 

 current. The older theory of electrolysis assumed that the two 

 components of the electrolyte were torn from each other by the 

 force of the current. According to the modern doctrine, on the 

 other hand, the assumption is that the separation of the ions 

 begins, and in all cases of good electrolytes is carried to a con- 

 siderable extent when the electrolyte is dissolved in water or other 

 appropriate liquid. Take the case of common salt for example ; 

 when dissolved in water this theory requires us to believe that 

 a considerable proportion of the salt is no longer sodium chloride, 

 the molecules of which are each constituted of an atom of sodium 

 combined with an atom of chlorine as in the original solid salt. 

 It is believed on the contrary that most of the molecules are 

 broken up into ions of positive sodium, represented by the 



+ 

 symbol Na, and ions of negative chlorine, 01. These ions move 



