The Chemical Relations of Water 299 



prevented from again oxidising the iron. In Experi- 

 ment 130, the steam was in excess, and 1 the hydrogen 

 formed was removed from the tube by the pressure 

 of that portion of the steam which escaped without 

 reacting on the iron. Such reactions as this are termed 

 " reversible " reactions, and the equations representing 

 them are written thus : 

 Iron + hydrogen oxide 



^hydrogen + triferric tetr oxide. 

 The two arrows with their points oppositely directed 

 denote that the reaction will take place in the forward 

 or backward direction according to the conditions under 

 which the action is carried out. 



Exercise for Student. 



Try to find other cases of reversible reactions in the work already 

 performed. Write the equations representing any that may be dis- 

 covered so as to show their reversible nature. 



205. The relation between the volumes of hydrogen 

 and oxygen which combine with 

 one another may be determined by 

 means of an instrument known as 

 the U-tube eudiometer. This is 

 shown in the annexed figure. A 

 measured volume of oxygen is 

 introduced into the closed gradu- 

 ated limb, the volume being read 

 off after adjusting the quantity 

 of mercury until it stands at the 

 same level in both limbs. Hydrogen 

 is next introduced, usually in con- 

 siderable excess of the amount 

 needed to combine with the oxygen 

 previously passed in. The combined volume of the two 



