io6 CHEMISTRY FOR AGRICULTURAL STUDENTS 



combustible substances, and therefore of supporting their 

 combustion. Ignite a piece of magnesium and plunge it into 

 a bottle of the gas. Note that the magnesium continues to 

 burn, although apparently with some difficulty ; that the white 

 oxide, magnesia, is produced, together with a black substance 

 Pour a little hydrochloric acid into the bottle to dissolve the 

 magnesia, filter off the black particles, wash with water, and 

 dry. Heat the black substance in a dry test-tube. Does it 

 burn and produce carbonic acid gas ? If so, it is carbon. 

 Represent the reaction by an equation. 



Carbonic acid gas even supports the combustion of carbon 

 itself. Attach the carbonic acid apparatus to a piece of 



1 



Fig, 44. 



combustion tubing, flask, and bent tube, as figured. Fill the 

 combustion tube with lumps of dry charcoal, and place a 

 solution of caustic potash (i gram to 3 c.c. of water) in the 

 flask through which the products of combustion pass. Heat 

 the combustion tube strongly, and then pass a very slow current 

 of carbonic acid gas through the apparatus. Note that the 

 gas after passing over the heated charcoal is no longer 

 completely absorbed by the caustic potash, as would be the 

 case if the carbon dioxide were unchanged. Collect some 



