CH. xxxvi. ELECTROLYSIS. 365 



made of agate, and afterwards of pure gold, because he 

 found that the clay of the china cups was acted upon by the 

 current. Yet, in spite of these and many other precautions, 

 the acid and the alkali still continued to appear. Then he 

 used water which he had evaporated very slowly, instead 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 

 ptire oxygen at one pole and hydrogen at the other. 



By this experiment Davy not only confirmed Cavendish's 

 discovery that pure water is made of 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 

 Bergman n's method of tests, for when you drive one ele- 

 ment 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 decomposed. For several reasons, how- 

 ever, Davy believed that it would be possible to reduce them 



