CH. XXVII. HYDROGEN AND OXYGEN. 231 



he wanted in order that he might be able to think without 

 interruption. 



In the year 1766 he read a paper before the Royal 

 Society upon a gas which he called * inflammable air,' 

 because it burst into a flame whenever a light was brought 

 near it, and also because he believed it to be the cause of 

 the explosions which so often take place in mines. He 

 obtained this gas by pouring sulphuric acid and water upon 

 zinc, iron, or tin, and then collecting the bubbles as Black 

 had done (see Fig. 38, p. 227). But when he began to make 

 experiments with this gas he found it very different from 

 Black's ' fixed air.' It is true that a candle would not burn, 

 nor could animals breathe in it j but when a light was brought 

 near it, it took fire and burnt with a pale blue flame inside 

 the bottle. Then instead of being heavy like * fixed air,' it was 

 lighter than the atmosphere, and for this reason it was soon 

 used for filling balloons. It had also another remarkable 

 peculiarity, that when mixed with air in a bottle, it exploded 

 with a loud noise directly a light was brought near it, leaving 

 drops of moisture inside the bottle. Cavendish did not 

 understand the cause of this explosion at first, but in 1784 

 (after Priestley had discovered oxygen) he mixed pure oxygen 

 and hydrogen in a closed vessel, and lighted them by an 

 electric spark, and then he made the great discovery that 

 these two gases, when lighted, rush together and form water ^ 

 which is therefore a compound substance made of oxygen 

 and hydrogen. 



Oxygen discovered by Priestley in 1774, and by Scheele 

 in 1775. — The next gas discovered was oxygen, the most 

 common and the most useful of all the substances in our 

 globe. It was discovered independently by two men — 

 Priestley, a dissenting minister at Leeds, and Scheele (born 



