CH. xxxvn. ELECTROLYSIS. 393 



of distilling it, because he found that distilled water carried 

 away some salt with it. When he had done this the acid 

 was weaker, but the alkali was as strong as ever. At this 

 point it occurred to him that the alkali might perhaps come 

 out of the air, so he put his gold cups of water under an 

 air-pump, and completely exhausted the air, filling the pump 

 with hydrogen to make quite sure that no other gas could 

 be left in. When he had tried this several times and made 

 it perfect in every way, he succeeded at last in getting nearly 

 pure oxygen at one pole and hydrogen at the other, while the 

 water decreased in the cups. 



By this experiment Davy not only confirmed Cavendish's 

 discovery that pure water contains nothing but hydrogen and 

 oxygen, but he also established a totally new method of 

 analysing substances, and finding out the materials of which 

 they are composed This method was in some ways more 

 certain than Bergmann's method of tests, for when you drive 

 one element out by putting another in its place, you have 

 some difficulty in finding out exactly what has happened ; but 

 when a substance is decomposed by electricity you literally 

 take it to pieces, and see the elements of which it consists. 



Discovery of Potassium and Sodium. Having suc- 

 ceeded in the case of water, Davy now went on to try the 

 effect of the electric current on other bodies, and the first which 

 he took were common potash and soda, which had always been 

 supposed to be simple substances, which could not be decom- 

 posed. For several reasons, however, Davy believed that it 

 would be possible to reduce them into more than one sub- 

 stance. So he heated some pure potash in a spoon until it 

 was quite liquid, and fastening the two ends of the spoon to 

 the wires of a battery > he sent an electric current through it. 

 After a little while the potash began to be agitated, and to rise 

 up in bubbles, and then there came to the surface beautiful 



