THE THIRD CONSTITUENT OF THE AIR 25 



In what proportion are carbon and oxygen combined in 

 carbonic acid gas? In order to determine this, charcoal is 

 burnt in a hard glass tube in a current of oxygen.* The 

 carbonic acid gas produced together with the excess of 

 oxygen is passed through a solution of caustic potash, and 

 finally through a tube containing lumps of caustic potash, 

 this being a substance which, like lime, absorbs carbonic 

 acid gas, but allows the excess of oxygen to escape. The tube 

 containing the charcoal, and the bulbs and tube containing the 

 caustic potash, are weighed before and after the experiment. 

 The former gives the carbon consumed, the latter the carbonic 

 acid gas produced ; the difference between the two will be the 

 oxygen combined with the carbon. 



The following numbers were obtained from an experiment : — 



Tube and charcoal before experiment = 78.65 

 „ after „ = 78.51 



Carbon burnt = .14 



Potash bulb and tube before experiment = 96.19 



„ „ after „ = 96.70 



Carbonic acid gas produced ^ .51 



From these figures calculate the proportion of carbon and 

 oxygen per cent, of carbonic acid gas, and also calculate the 



* This experiment is too difficult for elementary students, and therefore 

 only a bare outline of the method is given. It should, however, be per- 

 formed by the teacher with the aid of his students. To ensure a correct 

 result, the oxygen should be dried by passing through a U tube containing 

 pumice soaked with oil of vitriol, the charcoal must be previously heated 

 in a current of chlorine to eliminate every trace of hydrogen, and the 

 products of combustion of the charcoal must be passed through a heated 

 tube of oxide of copper, to ensure complete oxidation before passing into 

 the solution of caustic potash which should be contained in properly con- 

 structed potash bulbs. Two or three experiments can be made with the 

 same charcoal and potash, one after the other. 



