STORAGE OF GASES. 165 



(carbon dioxide) being much heavier than air (as illustrated by 

 Expt. 100), may be collected by displacement by simply passing 

 the end of the delivery tube of an appropriate generator down to 

 the bottom of a jar standing on a table mouth upwards. In the 

 case of the ammonia, the lighter gas accumulates in the inverted 

 jar, and thus gradually displaces the air originally contained in it, 

 the ammonia being unable to rise through the solid glass ; in the 

 case of the carbon dioxide, the heavier gas accumulates in the erect 

 jar in exactly the same way as water would do if run in gradually 

 by a pipe. 



If the position of the collecting jars be reversed in such experi- 

 ments, no gas at all will be retained ; thus if you attempt to collect 

 ammonia gas in a jar mouth upwards, all the ammonia will rise 

 out of the jar and escape into the air as rapidly as it passes in 



Fig. 73. Mercurial Trough. 



from the generator ; and, conversely, if you try to collect carbon 

 dioxide in a jar held mouth downwards, the jar will never become 

 properly filled, because the heavy gas will fall out of the jar as 

 quickly as it passes in, just as water would do under similar con- 

 ditions. In the same kind of way a jar of hydrogen (lighter 

 than air) held mouth upwards for a few seconds, allows all the 

 gas to float up, heavier air taking its place, so that no hydrogen 

 is left in the jar (Expt. 105) ; whilst a jar of carbon dioxide 

 held mouth downwards for a few seconds, similarly becomes 

 emptied of that gas, the relatively lighter air taking its place 

 (Expt. 101). 



For storing gases not readily soluble in water, gasholders are 

 used such as that shown in fig. 74. A copper drum, surmounted 

 by a reservoir for water, is connected with the latter by means of 

 two pipes, one of which passes down to the base of the drum, and 



