256 



J. M. Lewis : 



Fig. 11. 



examined. Immediately this is done, the gas should be 'cut off,, 

 the cock on the piston being provided for this purpose. The 

 piston is then gently drawn out of the barrel, and the solution 

 allowed to enter the vessel, and completely to cover the platinum 

 wire. Keeping the end of the vessel still below the surface 

 of the liquid, the contained fluid is ejected from the tube, allow- 

 ing a minute portion to remain in the capillary end. Another 

 portion is now drawn up and down the platinum wire, and again 

 ejected, after which the final portion is allowed to enter the 

 vessel in such a manner that the platinum wire makes a mini- 

 mum contact with the fluid. In* dealing with liquids which do 

 not contain carbon dioxide, it is not perhaps so necessary to> 

 leave a small quantity in the tube as described, since the ob- 

 ject of this is to retain the original hydrogen atmosphere, plus 

 carbon dioxide in equilibrium with that contained in the fluid. 

 The reason for " washing " the platinised wire with the fluid of 

 the final portion is, that in this way the contained oxygen is re- 

 duced by the hydrogen of the wire, and a steady e.m.f. is sooner 

 obtained, also the changing of the solution should enable the 

 COs in the final portion to be in equilibrium with that in the 

 hydrogen, and thus do way with any risk of diffusion from; 



