194 Messrs. St. Landau and Ed. Stenz on the 



III. Observations above 400° C ; influence of dissociation. 



The dissociation of iodine vapour was the subject of a very 

 thorough research of Bodenstein and Starck*. Denoting by 

 K a constant, by pi and pi 2 the pressures of the mono- 

 and diatomic iodine, by T the absolute temperature, they 

 obtained 



^L=K and log K= -~^ + 1-75 log T- 0-000416T + 0'422. 

 Pi 2 J- 



That formula, based on the thermodynamical theorem of 

 Nernst, is in good accord with the experiments. We applied 

 that formula in our calculations of the degree of dissociation 

 of the iodine vapour. 



The vapour we had to do with was kept at a pressure of 

 0'25 mm. mercury. The formula above permits us to calculate 

 K for different temperatures, and then to find the ratio x of 

 the number of dissociated molecules to the total number of 

 molecules which would be present were there not any 

 dissociation. 



Temperature ... £=500° C; 600°; 700°; 800°; 900°. 



^c^moL 6 }^ -^ °' 36 ; <> 76 ; °^ 0-99. 



The main difficulty of such observations is due to the fact 

 that every furnace heated above 500° C. emits so much light 

 that the weak light of fluorescence vanishes. We remarked 

 that a gas flame, when sufficiently supplied with air, is pale 

 enough to permit observations to be made, the bulb with the 

 vapour being in the flame. A great Meker furnace was fed 

 with the gas exhausted from the gas pipes by a Graede 

 rotatory oil-pump ; the gas passed through a vessel of 20-30 

 litres capacity to reduce oscillations of pressure. The lower 

 part of the burner was supplied with a ring to regulate the 

 quantity of air drawn into the burner. The flame had a 

 length of 25 cm. and a diameter of 4 cm., although the gas 

 pressure in the pipes was very low. It may be mentioned 

 that such a flame was found to be useful in many cases for 

 the glass-work ; it is longer and hotter than the blowpipe- 

 flame. 



The quartz apparatus we employed was similar to that 

 shown in tig. 1 ; the part corresponding to A was a sphere of 

 3J cm. in diameter, the part B was likewise of quartz, the 



* Zeilschriftf. Hiektrochemie, vol. xvi. p. 961 (1910). 



