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PRINCIPLES OF CHEMISTRY 



The first part of the gas evolved should not be collected, as it is 

 mixed with the air originally in the apparatus. This precaution 



phuric acid in the other. The neck of the former is closed by a cork, which is fitted with 

 a gas-conducting tube with a stop-cock. If the two bottles are put in communication 

 with each other and the cock be opened, the acid will flow to the zinc and evolve hydro- 

 gen. If the cock be closed, the hydrogen will force out the acid from the bottle contain- 

 ing the zinc, and the action will cease. Or the vessel containing the acid may be placed 

 at a lower level than that containing the zinc, when all the liquid will flow into it, and in 

 order to start the action^the acid vessel may be placed on a higher level than the other, 

 and the acid will flow to the zinc. Such an arrangement presents the simplest form of a 

 continuously-acting apparatus, which is of great use in chemical work. It can also be 

 employed for collecting gases (as an aspirator or gasometer). 



In laboratory practice, however, other forms of apparatus are generally employed for 



FIG. 22. Constant-acting aspirator. The tube d should be long (over 32 feet). 



exhausting, collecting, and holding gases. We will here cite the most usual forms. An 

 aspirator usually consists of a vessel furnished with a stop-cock at the bottom. A stout 



