THOMPSON. — ON THE EQUILIBRIUM OF THE SYSTEM. 449 



two causes, both the decomposition of carbide and to volatilization of 

 some impurity in the lime or carbon. The best evidence that the car- 

 bide does not break up at 1475° and does break up at 1525° is that 

 equilibrium could be measured at the former but not at the latter 

 temperature. Attention was called to the possibility of lime itself 

 being somewhat volatile at 1500°, since a piece of Merk's lime heated 

 at the melting point of platinum for an hour also produced a layer of 

 white powder on the walls of the furnace. 



As Experiments 1 and 2 were carried out at temperatures equally 

 above and below the temperature in experiments 6 and 7, the average 

 of these four may be taken, with the result 



p co at 1475°C = 0.82 ± .02 mm. 



Through these results were obtained from the same side of the equilib- 

 rium, different amounts of carbon monoxide were present at the 

 beginning in each case, which makes the evidence that equilibrium had 

 been reached conclusive. 



From this result, the pressure obtained in Experiment 8 at a tem- 

 perature 30° lower may be checked by the integrated van't Hoff 

 equation : 



4. 5 7lo glo g = «(l-i) 



where p% and p\ and the pressures of carbon monoxide corresponding 

 to the absolute temperatures 7\ and T 2 and Q is the heat absorbed by 

 the reaction, when it proceeds from left to right. Q has been calcu- 

 lated 14 to be 121000 calories at room temperature, with a negative 

 temperature coefficient of 3.3 calories per degree. 



Therefore Q = 121000 — 3.3 t, 



where t equals centigrade degrees above room temperature, which for 

 high temperatures may be considered as degrees above zero. For 

 1460° C. Q therefore equals 116000 calories. Substituting in the 

 above equation the absolute temperatures corresponding to 1475° 



and 1445°, the value of — comes out 1.79. The ratio between the 



1\ 

 pressures found by experiment is 1.86, which is very satisfactory 

 agreement. 



If the pressure at 1270° is calculated from that at 1475°, using the 

 value of Q corresponding to the mean temperature 1370°, the result is 



14 Trans. Am. Electrochem. Soc, 1909, 15, 197. 



VOL. XLV. — 29 



