Water- Vapour in contact with Hot Platinum Wire. 633 



The globe was completely evacuated, and then filled with 

 water- vapour by opening a fca beneath it which communicated 

 with a smaller bulb containing pure distilled water. 



When the large globe was completely filled with vapour, 

 the wire was heated to the required temperature, which 

 was kept as constant as possible till equilibrium had been 

 established. The current which heated the wire was then 

 cut off, and the whole contents of the bulb pumped off through 

 a freezing-mixture in order to condense the water-vapour. 

 The electrolytic gas was collected, and estimated by explosion, 

 when it was always found to contain a minute excess of 

 hydrogen. This excess appeared to be due to the absorption 

 of a trace of oxygen by the mercury in the fall-tube of the 

 pump. 



The temperature of the wire was determined from its 

 resistance by the following formula: — 



t-6 = 8 



U— Y- 



— 1 



100J 



6: 





100. 



1*54 was the value adopted for 8. 



Experiments were made with two different wires, in order 

 to see whether they gave concordant results. 



In the following tables the percentage decomposition of 

 water-vapour at the various temperatures is given. 



The values at the pressure of the water-vapour were deter- 

 mined experimentally, while those at 760 mms. pressure were 

 calculated from them. 



Wire No. I. 



Temperature 

 absolute. 



Per cent, decompo- 

 sition at the pressure 

 of water-vapour. 



Calculated per cent. 



decomposition at 

 760 mms. pressure. 



1220° C 



1540 ., 



00038 



00573 



0-0884 



0118 



0153 



0-00108 



0015 



00246 



0-0317 



004 



1640 „ 



1700 „ 



1760 „ 





