Solution of A tmospheric Nitrogen and Oxygen by Water. 393 



Of these it seemed that No. 1 was the most important ; 

 No. 3 was extremely small; while subsequent experiments 

 have shown that No. 2 is negligible, with a narrow tube, 

 such as was used in these experiments. 



In order to eliminate these errors it was decided to make 

 a new form of apparatus, suitable for use ^ith a pure gas, 

 such as nitrogen or oxygen; and to work with air-free 

 water. 



(a) Preparation of Air-free Water. 



In order to ensure that the water was air-free, it was 

 necessary to boil it in the vacuum of the mercury pump, and 

 then transfer it to the experimental tube without allowing it 

 to come in contact with the air. At first it was thought 

 that it would be sufficient to heat the water until its vapour- 

 pressure was great enough to force it over into the tube, but 

 it was found that this necessitated much too high a tempera- 

 ture. It was decided to displace the water with mercury, 

 but this introduced such a narrow tube between the flask 

 and condenser that the condensed vapour blocked it. 



The difficulty was finally solved by providing a second 

 tube to allow the water condensed to flow back into the 

 flask. It was thus possible to boil the water in vacuo as long 

 as might be necessary, without any appreciable loss by 

 evaporation. 



The diagram (fig. 4) shows the form of apparatus used, 

 and the mode of operation is as follows : — The water in A is 

 heated to a fairly high temperature by means of a water- 

 bath, and the mercury pump is then worked until a very low 

 pressure is reached. In this way most of the gas is extracted 

 in the first violent ebullition, and the remainder is removed 

 by continued boiling under the reduced pressure. During 

 the latter part of the operation the water bumps very 

 violently, with the result that some of it is thrown over into 

 the vessel E ; but this returns to the flask immediately by 

 the tube i?, as also does any water dripping from the con- 

 denser. When all the air is extracted, the pinch-cocks B 

 and C are closed, and I) is opened, when the mercury flows 

 in and displaces the water into the experimental tube, which 

 has been previously filled with mercury, and connected to 

 the flask A by another tube not shown in the drawing. 

 All the rubber stoppers used were protected from leakage by 

 mercury traps, and the rubber tubing was varnished with 

 shellac to prevent diffusion. 



