118 PRINCIPLES OF CHEMISTRY 



a retort or flask is passed through the tube D, into the internal porous 

 tube T. This steam on entering the red hot space is decomposed into 

 hydrogen and oxygen. The densities of these gases are very different, 

 hydrogen being sixteen times lighter than oxygen. Light gases, as \ve 

 saw above, penetrate through porous surfaces very much more rapidly 

 than denser gases, and therefore the hydrogen passes through the pores 

 of the tube into the annular space very much more rapidly than the 

 oxygen. The hydrogen which separates out into the annular space 

 can only be collected when this space does not contain any oxygen. 

 If any air remains in this space, then the hydrogen which separates 

 out will combine with its oxygen and form water. For this reason a 

 gas incapable of supporting combustion for instance, nitrogen is pre- 

 viously passed in the annular space. Thus the nitrogen is passed 

 through the tube c, and the hydrogen, separated from the steam, is 

 collected through the tube c', and will be partly mixed with nitrogen. 

 A certain portion of the nitrogen will penetrate through the pores of 

 the unglazed tube into the interior of the tube T. The oxygen will 

 remain in this tube, and the volume of the remaining oxygen 

 will be half that of the volume of hydrogen which separates out from 

 the annular space. Part of the oxygen will also penetrate through 

 the pores of the tube ; but, as was said before, a much smaller quan- 

 tity than the hydrogen, and as the density of oxygen is sixteen 

 times greater than that of hydrogen, the volume of oxygen which 

 passes through the porous walls will be four times less than the volume 

 of hydrogen (the quantities of gases passing through porous walls are 

 inversely proportional to the square roots of their densities). The 

 oxygen which separates out into the annular space will combine, at a 

 certain fall of temperature, with the hydrogen ; but as each volume of 

 oxygen only requires two volumes of hydrogen, whilst at least four 

 volumes of hydrogen will pass through the porous walls for every 

 volume of oxygen that passes, therefore, part of the hydrogen will 

 remain free, and can be collected from the annular space. A corre- 

 sponding quantity of oxygen remaining from the decomposition of the 

 water can be collected from the internal tube. 



The decomposition of water is produced much more easily by a 

 method of substitution, taking advantage of the affinity of substances 

 for the oxygen or the hydrogen of water. If a substance be added to 

 water, which takes up the oxygen and replaces the hydrogen then we 

 shall obtain the latter gas from the water. Thus with sodium, water 

 gives hydrogen, and with chlorine, which takes up the hydrogen, 

 oxygen is obtained. 



Hydrogen is evolved from water by many metals, which are capable 



