The Chemical Relations of Water 



287 



upon the metal to go on for about two minutes. 



Before attachment, it should be deflated as far as 



possible by gentle pressure between 



the hands. If the gas does not 



exert sufficient pressure to inflate 



the balloon properly, cover the mouth 



of the funnel with the ball of the 



thumb. The increased pressure thus 



produced will be generally found to 



effect the filling without further 



trouble. When full, release the 



balloon, which will quickly rise to. 



the ceiling. After some time, the 



hydrogen will slowly escape and it 



will descend and may be preserved 



for future use. 



198. Other metals may replace 

 the zinc in the preparation of hy- 

 drogen, and other acids may be used 

 instead of the hydrochloric acid. 

 (Nitric acid is unsuitable, however.) 

 This may be tested by putting 



Fig. 94. 



small quantities of iron, magnesium, and tin at the 

 bottom of separate test-tubes, and pouring various 

 acids, such as sulphuric, hydrochloric, and acetic, 

 upon samples of each of them. In most cases, vigorous 

 effervescence of the liquid indicates the escape of the 

 hydrogen, but where this does not occur, gently warming 

 the tube, or substituting a weaker or stronger sample 

 of acid, will cause the hydrogen to be set free. 

 Magnesium + hydrogen chloride 



= magnesium chloride + hydrogen. 

 Iron + hydrogen sulphate 



= iron sulphate + hydrogen. 



