The Chemical Relations of Water 283 



later, the gas is much less dense than air and has 

 entirely escaped from the jar. 



If the iron used was quite clean and bright at the 

 beginning of the experiment, it may be examined after 

 the tube has cooled, when it will be observed that it is 

 now covered with a film of a blackish substance. This 

 is an oxide of iron called " triferric tetroxide ", or 

 magnetic oxide of iron. 



Iron + hydrogen oxide = triferric tetroxide + hydrogen. 



1 96. Hydrogen is less chemically active than oxygen 

 and no simple and convenient method is available 

 whereby the hydrogen of water may be readily re- 

 moved, leaving the oxygen in the free state. To obtain 

 a supply of oxygen from water, recourse may be had to 

 a method whereby electrical energy is transformed into 

 chemical energy with the final result that water is 

 separated into its two elements. 



EXPERIMENT 131. The apparatus shown in the 

 figure is known as Hofmann's voltameter. The two 

 side tubes are filled with water, to which a little sul- 

 phuric acid has been added, by pouring this liquid into 

 the bulbed reservoir, the taps at the top of each side 

 tube being left open while this is done. When enough 

 liquid has been poured in to fill the tubes exactly, the 

 taps are closed. The wires from a battery of two or 

 more Daniell or Bunsen cells, or, if it be available, 

 from the electric light supply, are then connected to 

 the loops of platinum wire near the bottom of each side 

 tube. (If a supply from the electric mains is used, 

 a lamp resistance should be inserted between the loops 

 and the wall plug, as indicated in the figure.) As soon 

 as the attachment is made, bubbles of gas are to be 

 seen ascending from each of the two platinum plates 



