X3ARBURETTED HYDROGEN. 279 



A. Because the lime re-imbibes the 

 carbonic acid of the air, which was 

 expelled by fire ; and the loose powder 

 again becomes as hard as the original 

 lime-stone. 



Q. Why is MORTAR adhesive ? 



A. When the carbonic acid is expel- 

 led, the hard lime-stone is converted into 

 a loose powder, which (being mixed with 

 sand and water) becomes a soft and 

 sticky plaster ; but, as soon as it is placed 

 between bricks, it imbibes carbonic acid 

 again, and hardens into lime-stone. 



CHAPTER XX. 



CARBURETTED HYDROGEN 

 GAS. 



Q. What is CHOKE-DAMP ? 

 A. Carbonic acid gas accumulated 

 at the bottom of wells and pits. It is 



Ba 2 



