22 CHEMISTR Y FOR A GRICUL TURAL STUDENTS 



students who, if possible, should use different specimens of 

 the red oxide. Should they have found the same combining 

 proportions, it must be concluded that mercury and oxygen 

 combine in definite proportions by weight. Subsequent ex- 

 periments will show whether this is true of the elements of other 

 chemical compounds. 



V. THE THIRD CONSTITUENT OF THE AIR 



Besides rusting and burning, another familiar change that 

 takes place in air is the conversion of lime into chalk. 

 Expose some solution of lime (" lime water ") in a beaker to 

 the air. Note that in a few minutes it becomes covered with 

 a white scum of chalk, chalk being distinguished from lime by 

 being insoluble in pure water. Repeat the experiment, but 

 use a stoppered bottle filled with air instead of an open 

 beaker. Note that the amount of chalk now formed is 

 extremely small, and that no appreciable absorption of the air 

 in the bottle occurs. These experiments show that there is a 

 constituent in the air which converts lime into chalk, but that 

 it is present in very minute quantities, and that the oxygen and 

 nitrogen of the air are not concerned in the change. 



Since the product of combustion of charcoal or carbon — the 

 oxide of carbon — is a gas, it is very probably a constituent 

 of the air, and it may be this which converts lime into chalk. 

 Burn some charcoal in a bottle of oxygen (supplied), and test 

 the gas produced with lime water. Should chalk now be 

 formed, this third constituent of the air will have been 

 identified as the oxide of carbon. 



Re-examine the properties of this gas as prepared by the 

 combustion of charcoal in oxygen. Try whether it is able or 

 unable to support the combustion of a taper. Leave a bottle 

 of the gas open, and test it from time to time with a burninfr 



