The Chemical Relations of Water 295 



tap. The pressure of the liquid in the centre tube 

 and in the top bulb forces the air out of the middle bulb 

 through the tap. The acid from the bottom bulb rises 

 through the neck between that and the middle bulb 

 and thus comes in contact with the zinc. Hydrogen 

 is immediately evolved and issues from the tube 

 connected with the tap. Allow the action to continue 



Fig. 98. 



for a short time in order to expel as much air as possible 

 from the middle bulb. Close the tap. Hydrogen is 

 still generated by the action of the acid remaining in 

 contact with the zinc, but can no longer escape. Hence 

 the pressure of the gas in the middle bulb increases 

 and, as a result, the acid is forced out of the middle 

 bulb and returns to that at the top, the action ceasing 



