226 EIGHTEENTH CENTURY. PT. in. 



shown that the atmosphere could be separated into two 

 gases, but his experiments had been passed over and for- 

 gotten; and though Dr. Hales, at the beginning of the 

 eighteenth century, had collected several gases, he had not 

 distinguished them from air. The fact was that Stahl's 

 imaginary ' phlogiston,' which was supposed to pass out of 

 burning and breathing bodies into the air, was a constant 

 source of confusion, and led men away from the truth. 



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 Black, 

 1756. The first step was made by a Scotch physician 

 named Black, who was born 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 

 without any change unless you add a little acid to the 

 water, and then the limestone will effervesce, and bubbles 

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

 of the same limestone and burn it in a fire, it turns into 

 a powder 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 you may see for yourself if you watch bricklayers making 

 mortar by the roadside. This complete change in the lime- 

 stone, 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 



