16 THE DISCOVERY OF ARGON AND HELIUM. 



nitrogen gas passed round and round over the magnesium, then 

 finally there is a residue left, and that residue is argon. I have a 

 slide here showing the various densities of these gases that Lord 

 Rayleigh obtained. 



. The way in which nitrogen was absorbed afterwards, when it 

 got down to a small bulk, is shown on the next slide. The water 

 in the vessel contains caustic soda. There are two platinum 

 points connected by wires to an apparatus for making sparks. If 

 gas containing the last traces of nitrogen, which is difficult to be 

 got rid of by means of magnesium, is introduced into such an 

 apparatus with oxygen, and the electric spark be allowed to pass 

 between the two points b and d^ the nitrogen will combine with 

 the oxygen and form a substance which can be absorbed in the 

 soda solution. That is the way of getting rid of the last traces of 

 nitrogen ; and there are then left only argon and oxygen ; the 

 oxygen can be absorbed by any ordinary re-agent, such as phos- 

 phorus. 



The next sHde shows a rather more elaborate apparatus for 

 dealing with a larger quantity. The electrodes here dip down so 

 as to touch the surface of the liquid soda. That gives a larger 

 space, and combination takes place more readily. To keep it cool 

 Lord Rayleigh had occasionally a spray of soda running up to 

 cool the top of the vessel. It is an improved apparatus for work- 

 ing on larger quantities. 



The next slide shows a still further improvement in the appa- 

 ratus. This is one in which the electric spark is allowed to play 

 between the two tubes. The section shows the bath, which is 

 cooled by means of water. 



Next comes a diagram which shows the apparatus that Profes- 

 sor Ramsay used first of all for the absorption of nitrogen. It 

 contains various absorbents for absorbing nitrogen and other 

 gases present in ordinary atmospheric nitrogen. Starting at A by 

 letting water run in, the gas is sent through the whole apparatus 

 into B^ and when it gets there it can be sent back again into A^ 

 and so it may be passed backwards and forwards. In the various 

 parts of the apparatus are phosphorus pentoxide to dry the gas, 

 copper to get rid of the oxygen, copper oxide to get rid of the 

 hydrogen, soda and lime to get rid of the acid vapours, and 



