96 ELEMENTS AND COMPOUNDS 



of the metal platinum (c/. 9), and they have tips of platinum foil. 

 We call the ends of the wires the poles of the battery. If we were to 

 put the two platinum poles together, the current would have a com- 

 plete passageway, or circuit, without going through the dilute acid. 

 But if we keep the poles apart, the current is compelled to pass 

 through the dilute acid. In the language of the electrician, the dilute 

 acid makes, or closes, the circuit. 



While it is carrying the current from one pole to the other, the 

 dilute acid is changed chemically (c/. 98). What we see is that 

 bubbles of gas arise from the poles. We can collect the gas by placing 

 over each pole a test tube filled with some of the dilute acid. We then 

 see that one test tube collects gas about twice as rapidly as the other. 

 If we put a burning splinter into the gas that is collected the more 

 slowly, the splinter burns more brightly than in air, and if the splinter 

 is merely glowing, it will burst into flame. The gas collected in this 

 tube is oxygen. If we bring a flame near the other gas, the gas takes 

 fire with a slight "pop," or explosion, and then burns with an almost 

 invisible blue flame. This gas is hydrogen. It is so called because 

 it is a part of water. The Greek word for water ' ' hydor " appears 

 in many other English words, such as hydra, hydrant, hydraulic, etc. 



101. Elements and Compounds. Water is so hard 

 to decompose by heat that men were unable to learn its 

 real nature until 1781. In that year Cavendish, who had 

 prepared hydrogen in 1766, burned hydrogen, and ob- 

 tained water. The decomposition of water by the electric 

 current was first carried out in 1800. The question now 

 arises : ' l Can the hydrogen and the oxygen obtained from 

 water be divided up into other substances?" The answer 

 is that they have never been divided by any method used 

 for the purpose. A substance like water, which is not a 

 single kind of matter, but has at least two kinds of matter 

 in it, is called a compound. A kind of matter which we 

 have never been able to break up is called a simple 



