Oxygen 



259 



Phosphorus pentoxide + hydrogen oxide = hyd rogen metaphosphate 



(water) (metaphosphoric acid) 



Sodium oxide + = sodium hydroxide 



(caustic soda) 



Bodies like caustic soda, which, when dissolved in 

 water, are capable of producing the reverse change in 

 the colour of litmus to that effected by the action of 

 acids, are termed "alkaline" (from the Arabic al, the, 

 and qali, the ashes of a plant, known as glasswort, from 

 which soda was first obtained). 



Fig. 83. 



177. Substances which are not looked upon 

 ordinarily as combustible may often be burnt in oxygen. 

 Iron is used in the construction of fire-grates and for 

 many other purposes where strength and resistance 

 to the action of heat are desirable properties. That 

 it would be no longer of service to man in this way 

 if the atmosphere consisted of oxygen alone is demon- 

 strated by 



EXPERIMENT 118. To the end of a piece of watch 

 or clock spring affix a short length of wax-taper by 



172 



