70 



coming from a hydrate, while the other is not. The settling of one 

 product as a precipitate, while the other stays in solution, is, as we 

 shall see (pp. 127, 129), another common way in which the separa- 

 tion occurs. 



When iron and water are heated in a closed vessel, the hydrogen 

 and oxide of iron which are produced (p. 51) react with one another 

 to give back water and iron. 



Iron + Steam + Hydrogen + Magnetic oxide of iron. 



Hydrogen Iron 



Oxygen Oxygen 



In a closed vessel we could never use this method for preparing 

 any quantity of hydrogen. To prepare hydrogen by this action 

 we must leave the tube open, and let the steam sweep the hydrogen 

 out. This separates it effectually from the oxide of iron, and 

 prevents the reversal of the action. 



Devices depending on mechanical principles like this are con- 

 tinually used in chemistry for securing easy methods of prepar- 

 ing substances. 



The reader can now answer for himself the question why we are 

 able to prepare oxygen by heating mercuric oxide (p. 15), in spite 

 of the fact that the action is reversible (p. 29). 



Exercises. 1. If 50 c.c. of hydrogen and 37 c.c. of oxygen are 

 exploded in a closed tube, which gas remains and what volume 

 of it is left? 



2. Why do not bubbles of steam ordinarily form in water and 

 rise through it at temperatures below 100? 



3. How many calories of heat would be required to change 5 

 grams of ice at into steam at 100? 



4. How could one find out how much solid matter was dissolved 

 in a sample of water? 



5. Define : filtration, distillation, efflorescence, chemical equilib- 

 rium. 



