THE BAROMETER. 



297 



The gas, passing through the tube, rises in bubbles through the mercury in the 

 receiver, and lodges at the top, and, by continuing this process, the whole of 

 the mercury will at length be expelled from the receiver, and its place filled 

 with the gas. In this manner gases of various kinds may be preserved out of 

 contact with the atmosphere, and the same shelf may be furnished with several 

 holes, and may support a number of different jars. 



The gasometer used in gas-works is constructed on the same principle, only 

 on a different scale. When used for great supplies of gas, such as are neces- 

 sary for the illumination of towns, these vessels are constructed of a very large 

 size, and are immersed in pits lined with cast-iron, and filled with water. It 

 is clear that all which has been just explained will be equally applicable, what- 

 ever be the liquid used in the cistern, 'and for different gases it is necessary to 

 use different liquids, since the contact with particular liquids will frequently 

 affect the quality of the gas. The peculiar gurgling noise which is produced 

 in decanting wine arises from the pressure of the atmosphere forcing air into 

 the interior of the bottle. In the first instance, the neck of the bottle is com- 

 pletely filled with liquid, so as to stop the admission of air. When a part of 

 the wine has flowed out, and an empty space is formed within the bottle, the 

 atmospheric pressure forces in a bubble of air through the liquid in the neck, 

 which, by rushing suddenly into the interior of the bottle, produces the sound 

 alluded to. This effect is continually repeated so long as the neck of the bot- 

 tle continues to be choked with the liquid. But as the contents of the bottle 

 are discharged, the liquid, in flowing out, only partially fills the neck ; and 

 while a stream of wine passes out through the lower half of the neck, a stream 

 of air passes in through the upper part. The flow in this case being continual 

 and uninterrupted, no sound takes place. 



The atmospheric pressure, acting on the surface of liquids, maintains air 

 combined with them in a greater or lesser quantity, according to the nature of 

 the liquid. If an open vessel, containing a liquid, be placed under a receiver, 

 and the air be exhausted, the air combined with the liquid will be immediately 

 set free, and will be observed to rise in bubbles to the top ; this effect will be 

 very perceptible if water be used, but still more so in the case of beer or ale. 



When liquor is bottled, the air confined under the cork is condensed, and 

 exerts upon the surface a pressure greater than that of the atmosphere. This 

 has the effect of holding in combination with the liquor air which, under the 

 atmospheric pressure only, would escape. If any air rise from the liquor after 

 being bottled, it causes a still greater condensation, and an increased pressure 

 above its surface. 



