CHEMISTRY OF THE EARTH 89 



hydrates, which are thus compounds contain- 

 ing three elements. For example, that well- 

 known substance called sodium hydrate, 

 chemically, or caustic soda, commercially, 

 of which our country manufactures millions 

 of tons annually for soap-making and many 

 hundreds of industrial uses, is very simply 

 composed of an atom of divalent oxygen 

 united to one atom each of univalent sodium 

 and univalent hydrogen. It might be 

 regarded as water with one of the two hydro- 

 gen atoms replaced by the metal sodium, 

 which possesses a stronger affinity for the 

 position. It is for this reason that caustic 

 soda is called a hydrate, and it may be actually 

 made by throwing a small piece of metallic 

 sodium on water when the water is decom- 

 posed, hydrogen discharged, and set fire to 

 by the heat energy evolved. This hydrogen 

 burns with the oxygen, re-forming water, 

 and the sodium present colours the flame 

 that intense yellow, which gives the 

 sodium spectrum referred to in the previous 

 chapter. 



Just in the same way as hydrogen was used 

 for measuring out the univalent group, 

 the oxygen atom may be used for measuring 

 out the divalent group. For example, the 

 metals calcium and copper each unite atom 



