Minor Constituents of Air 229 



are carried forward with the air-current, and absorbed 

 in the U-tube. 



The caustic potash retains the carbon dioxide by 

 combining with it, resembling lime-water in this respect. 

 It is, however, a much better absorbent than lime-water. 

 Thus: 



Potassium hydroxide + carbon dioxide 

 (Caustic potash) 



= potassium carbonate + hydrogen oxide. 

 (Carbonate of potash) (Water) 



The potassium carbonate, being a solid, remains in 

 the tube, while the water simultaneously produced, 

 together with that formed by the burning of the candle, 

 combines with the potash or with the calcium chloride 

 in the other limb of the U-tube, forming a solution 

 which remains on the exterior of the small lumps. 



After about one-quarter of an inch of candle has 

 burnt away, blow out the candle, and, after a few 

 seconds, turn off the tap. When the apparatus has 

 cooled to the temperature of the room disconnect the 

 aspirator, and again weigh the portion weighed before. 



It will be found that, although some of the candle 

 has disappeared, and has been, in everyday speech, 

 " destroyed ", the weight has increased. This increase 

 in weight is due to the oxygen which has entered into 

 combination with the constituents of the candle. 

 While it exists as free oxygen in the atmosphere, it 

 affects both pans of the balance equally, but when 

 combined with carbon and hydrogen, and absorbed by 

 the potash and calcium chloride, its weight is readily 

 obtained. 



157. The question next arises " Since the products 

 of combustion weigh more than the original substance, 



