

CH. xxvii. BLACK'S FIXED AIR. 225 



But the time had now come when these misty ideas were 

 to be dispelled, by the discovery of the four gases carbonic 

 acid, hydrogen, oxygen, and nitrogen. 



Discovery of ' Fixed Air,' or Carbonic Acid, by 1 

 Black, 1758. The first step was made by a Scotch | j ' 

 physician named Black, who was born of Irish and Scotch \ 

 parents at Bordeaux in 1728, and became Professor of 

 Chemistry at Glasgow in 1756. Here he made many 

 valuable experiments, and among other things he was very 

 anxious to find out why limestone altogether changes its 

 character when it is burnt If you take a piece of ordinary 

 limestone or chalk, and put it in water, it will remain witl - 

 out any change unless you add a little acid to the watei , 

 and then the limestone will effervesce, and bubbles will 

 begin to rise up from it. But if you take a piece of tho 

 same limestone and burn it in a fire, it turns into a powde : 

 .called quick-lime, which will no longer give out bubbles 

 when you pour acid upon it, but directly you mix it with 

 water it will swell up and become intensely hot, as may be 

 seen when bricklayers are making mortar by the roadside. 

 This complete change in the limestone, caused by merely 

 heating it, had been a great problem to chemists ; and Dr. 

 Black was still more puzzled by finding that the lime was 

 lighter after it had been burnt, although he could not \ 

 discover that it had lost anything except a little water, 

 which was not enough to account for the loss of weight. 



At last he remembered that Dr. Hales had driven air out 

 of substances, and collected it in bottles ; and he began to 

 consider whether the heat of burning might not have driven 

 some heavy kind of air out of the limestone, and so made it 

 lighter. To prove this he made the experiment which has 

 since been always used for making small quantities of car- 

 bonic acid gas. He put some pieces of limestone in the 



