HYDROGEN. 199 



indeed, be regarded as a general law, that the pres- 

 ence of acids promotes the formation of oxides, and 

 vice versa. 



492. ANOTHER METHOD. Hydrogen 



Give another J 



method ofpre- may also be made, by passing steam through 

 a heated gun-barrel, containing bits of 

 iron. Bundles of knitting needles are commonly em- 

 ployed for the purpose. The steam leaves its oxygen, 

 combined with the iron, and escapes as hydrogen gas. 



493. COMBUSTION OF HYDROGEN. Bring 



What i* pro- , , 



duced by the a dry, cold tumbler, over a burning jet of 

 combustion of hydrogen. The vessel will soon become 



hydrogen f 



moistened on the interior. The water 

 thus produced, is a result of the combination of hydro- 

 gen with oxygen of the air. But for the cold surface, 

 with which it is brought into contact, it would have 

 escaped into the air as vapor. The composition of 

 water was shown in Part 1., (> 277,) by galvanic de- 

 composition. It is here demonstrated by combining 

 its elements, and thus reproducing it. Water is also 

 formed in the combustion of any substance containing 

 hydrogen as one of its constituents. The above expe- 

 riment may therefore be made with a lighted lamp or 

 candle, as well as with the jet of pure hydrogen. 



494. EXPLOSION OF MIXED OXYGEN AND 



How is an ex- 

 plosive mix- HYDROGEN. Allow oxygen to flow into an 

 turcprepared? i nverte( i ^ as directed in para- 

 graph 330, until it is one-third full. Fill it up 

 with hydrogen, collected as shown in Par. 489. 

 Cork the vial under water. It is now filled with 

 an explosive mixture, which may be fired by the 



