434 



Prof. J. Dewar and Mr. H. 0. Jones. 



[Mar. 3, 



true critical temperature. The presence of 4 — 5 per cent, of carbon 

 monoxide with its low critical temperature 128° abs. would be expected 

 to lower the temperature at which the meniscus disappears by 

 about 5° C. That this quantity of carbon monoxide might be present 

 at any time is shown by the following experiment : — 



Volume of tube, 1*35 c.c. 



„ nickel carbonyl left after heating to 193° C. = 0*437 c.c. 

 Amount of nickel deposited = 0*0147 gramme. 



„ carbon monoxide liberated = 0*0284 gramme = 5*02 

 per cent, of the residual nickel carbonyl. 



Hence it is very probable that the true critical temperature is about 

 200° C. 



Comparative experiments made with pure ethyl ether in similar 

 tubes, gave a critical temperature of 193° C. The accepted value for 

 the critical temperature for ether being 194°*5 C, the observed tem- 

 perature for nickel carbonyl cannot be far removed from the correct 

 value. 



The formula Tj = 0*66 T c , where T& is the absolute boiling point 

 and T c the absolute critical temperature, should be applicable to the 

 case of nickel carbonyl since it is applicable to ether, a liquid which 

 has a critical temperature of approximately the same value. Taking 

 the boiling point to be 43°*3 C, the critical temperature calculated 

 from the above relation would be 201 °*4 C, which agrees very well 

 with the value which was found as a probable minimum critical tem- 

 perature. 



Although it had not been surmised that nickel carbonyl could ever 

 stand heating above 100° C, nevertheless Mond and Nasini* calculated 

 the hypothetical critical temperature of nickel carbonyl from the 

 results of their experiments on its coefficient of expansion by means of 

 a formula given by Thorpe and Riicker and found the value 151° C. 

 In a similar manner Ramsay and Shields! calculated the critical 

 temperature from the temperature coefficient of the molecular surface 

 energy. The value deduced in this way is 182°*8 C. It therefore 

 appears that the hypothetical critical temperature calculated by either 

 of these methods falls considerably below its actual value. 



Rough indications of the critical pressure were obtained by intro- 

 ducing into the sealed tube containing the nickel carbonyl a small 

 tube of very fine bore, closed at one end and having a small globule of 

 mercury introduced at the other, to act as a manometer. The posi- 

 tion of the globule was observed at the ordinary temperature, and 

 again at the critical point. The volume of the air in the small tube 

 occupied about one-thirtieth of its original volume at the latter 



* ' Zeit. Pbys. Chem.,' 1891, vol. 8, p. 150. 

 f 'Jour. Chem. Soc.,' 1893, vol. 63, p. 1108. 



