LESSONS IN CHEMISTRY. Ill 



Some of the metals decompose water. Throw a small piece 

 of the metal potassium on water ; it takes fire, burning with a 

 fine violet flame floating about as a melted globule on the sur- 

 face of the water. The potassium unites with the oxygen in the 

 water, and the heat generated is intense enough to ignite the 

 hydrogen which has been set free, and is also sufficient to 

 vaporize some of the potassium which gives the violet color to 

 the flame. 



The metal sodium behaves much like potassium, only that the 

 heat is not quite sufficient to ignite the hydrogen. These experi- 

 ments are interesting, but for preparing hydrogen in quantity, a 

 simple process consists in passing steam over red-hot iron. The 

 iron combines with oxygen of the water, forming the black oxide 

 of iron (Fe 3 4 ) the hydrogen passes on and is collected in jars over 

 water. The equation is as follows : 4H 2 0, steam -f- Fe 3 = Fe 3 4 , 

 oxide of iron -f H 8 . While the decomposition of water by po- 

 tassium and sodium at ordinary temperatures is prompt and 

 spontaneous, iron must be heated to a red heat before it can sep- 

 arate oxygen from hydrogen. 



The most convenient method of preparing hydrogen for use 

 is by treating a metal, as zinc, with a strong acid. Put two 

 or three pieces of granulated zinc into a test tube and pour 

 upon them some dilute sulphuric acid. A brisk action takes 

 place immediately, gas being given off in abundance. After 

 the action has continued for a few moments, bring a lighted 

 match to the mouth of the tube and the gas takes fire with a 

 slight explosion, burning with a pale flame. In preparing the 

 gas in a larger way, use a wide-mouthed bottle with a cork hav- 

 ing two holes ; through one of these pass a glass funnel whose 

 stem is long enough to reach nearly to the bottom of the bottle; 

 through the other hole pass the delivery tube. Use zinc and acid 

 as before, being sure that the funnel tube extends below the sur- 

 face of the acid in the bottle. After the chemical action has con- 

 tinued long enough so that all of the air has been driven out of 

 the bottle, the gas may be collected over water as in the case of 

 oxygen. Acid may be added through the funnel if the action 



