COMMON COMPOUNDS OF OTHER ELEMENTS 277 



lyte. Water solutions of acids, bases, and salts are 

 all conductors of electricity; and in each case the sub- 

 stance in solution (solute) is decomposed by the electric 

 current. The decomposition always results in two parts, 

 one of which is carried to the positive electrode while the 

 other is carried to the negative electrode. For example, 

 the electrolysis of acids always gives hydrogen at the 

 negative electrode and the remainder of the compound 

 at the positive electrode. Acids, bases, and salts are the 

 only substances which in water solutions are electrolytes. 



A simple apparatus for showing the conductivity of 

 electrolytes is shown in Figure 238. The lamp, which 

 should be a small one, 

 is placed in series with 

 the platinum electrodes. 

 When a conductor of 

 electricity is placed in the 

 jar and the connection is 

 made with a battery of 

 sufficient size, the lamp 

 will glow. If only a few 

 cells are available, a telephone receiver may be substi- 

 tuted for the lamp. When the circuit is made or broken, 

 the receiver will click. In this way test the conductivity 

 of dry salts. 



Analysis of Chemicals. It is often quite important 

 to determine just what elements or compounds are present 

 in certain substances. The agricultural chemist analyzes 

 soils to determine what crops they are best suited to, 

 and what fertilizers are needed. Health officers analyze 

 foods of all sorts to determine their food value and also 

 to guard the public against harmful substances which 

 are occasionally found in them. The geologist analyzes 



FIG. 238. 



