678 



DECOMPOSITION OF WATER. 



red-hot at the point is introduced: it will be rekindled; 

 This is known as a characteristic test for oxygen, for 

 combustion, which is nothing else but combination of 

 substances with oxygen, is much more intense in pure 

 oxygen than in air which contains .the oxygen in a 

 diluted state, since 100 parts by volume of air contain 

 only 21 of oxygen, the remainder being nearly all 

 nitrogen, a gas which does not support combustion. 



If the two gases produced by the decomposition of water, instead 

 of being separately received, are allowed to mix after being disen- 

 gaged, the mixture, called oxy- 

 hydrogen gas, explodes with 

 great violence and a loud re- 

 port when a light is applied to 

 it. For this purpose a small 

 inverted funnel is placed over 

 the electrodes instead of the 

 tubes, and a narrow india- 

 rubber tube is slipped over the 

 end of the funnel, as shown in 

 section in fig. 344. The end 

 of the rubber tube is led into a 

 small shallow dish, best of 

 metal or wood, containing 

 water in which some soap has 



FIG. 344 (J real size). 



been dissolved. The surface will 



soon be covered with a little heap of bubbles, which may be 

 lighted by a burning match, but not until after the end of the rubber 

 tube has been withdrawn from the liquid, or otherwise the flame 

 may be communicated through the tube to the funnel, and the 

 apparatus shattered by the explosion. The report is very loud and 

 sharp, the sharper the smaller the bubbles in the heap, and the 

 bubbles are the smaller the narrower the india-rubber tube is. 

 Small bubbles may, however, be also obtained from a wide tube, if 

 the mouth of the tube, which dips under the liquid, be held in a 

 horizontal position and pressed with the finger against the bottom 

 of the dish, so that the gas escapes only through a narrow slit 

 which remains open between the tube and the bottom of the vessel. 

 For the mere decomposition of water, when it is not intended or 



