716 



Mr. A. Holt on the 



Amer. Chem. Soc. 1906, xxviii. p. 1357), but at 2000° the 

 percentage was 1/7 the amount obtained by these authors. 



The present communication contains an account o£ a 

 repetition of the author's previous experiments. 



It has not been found possible to determine the dissociation 

 of water vapour with any accuracy for temperatures above 

 1650°, as it then becomes very difficult to calculate the 

 temperature of a platinum wire from its resistance. The 

 author's already published numbers for temperatures above 

 this point are therefore of little value. 



The errors in the determination below 1650° arose almost 

 entirely from the fact that at the time the work was being 

 carried out the author did not realize that the temperature- 

 resistance curve for platinum approximated to a straight line 

 above 1300°, and could not be calculated from the Heycock 

 and Neville formula. 



It will be seen from the following table that if the old 

 temperatures are recalculated the values for the percentage 

 decomposition do not greatly differ from those of other 

 experimenters. The variation that remains results from a 

 number of minor causes which have been corrected in the 

 present research. 



Old Temp. 



Eecalculated 

 Temp. 



Per cent, decomp. 

 at 760 mm. deter- 

 mined experimentally. 



Per cent, decomp. 

 at 760 mm. calcu- 

 lated for new temp. 



1220° 



1350 



1500 



1540 



1560 



1630 



1640 



1220° 



1318 



1435 



1465 



1480 



1530 



1540 



0-00108 



0-00175 



0-00887 



0-015 



00158 



0-01842 



0-0246 



0001 



0003 



00102 



0-0158 



0-018 



0-027 



0031 



Although the method adopted by the author in the present 

 and the previous research resembles that of Langmuir in 

 that the decomposition of the water vapour in both is brought 

 about by an electrically heated platinum wire, yet it differs 

 in one important feature, namely, the wire is heated in a 

 known volume of vapour at low pressure until equilibrium 

 is established, instead of in a current of steam at atmospheric 

 pressure. 



The advantage of using water vapour at low pressure is 

 that the amount of decomposition increases rapidly as the 



