en. 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 w r ould not burn, 



nor could animals breathe in it ; 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 wafer, 



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 



