298 



THE BAROMETER. 



If the nature of the liquor be such as to produce air in considerable quan- 

 tity, this condensation will at length become so great as to force out the cork; 

 or, failing to do that, break the bottle. This is found to happen frequently 

 with beer, ale, or porter. The corks in such cases are tied down by cord or 

 wire. 



When the cork is drawn from a bottle containing liquor of this kind, the 

 fixed air being released from the pressure of the air which was condensed un- 

 der the cork, instantly makes its escape, and, rising in bubbles, produces effer- 

 vescence and froth. Hence the bead observed on porter and similar liquors 

 and the sparkling of champagne or cider. 



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