m 



Prof. Buff on the Relation between the 



The differences are not more considerable than would be 

 expected, and in no case exceed those which exist between 

 these recently obtained numbers and those deduced from the 

 earlier observations of Regnault*. A difference of o, l in tem- 

 perature signifies a variation of 0*2 millim. in the tension of 

 alcohol- vapour at temperatures between 10° and 20°, 3 millims. 

 between 70° and 80°, and as much as 15 millims. at a tempera- 

 ture of about 140°. With the exception of a few cases, the dif- 

 ferences between the two columns of numbers are not greater 

 than that which corresponds to o, l of temperature. A still 

 greater accordance would have been obtained if I had not assumed 

 that the tension at 100° was determined with the greatest accu- 

 racy, and that the boiling-point of alcohol must be somewhat 

 higher than it is according to Regnault's numbers. According 

 to the latter it is below 78°*3, whilst the most accurate determi- 

 nations have given 78°*4. 



The preceding formulae furnish the following numbers : — 



r. 



P millims. 



78-4 

 78-401 



78-5 



759968 

 760000 

 763-019 



Prom the numbers given for the tensions of ether-vapour I 

 obtain the following simple expression for a : 



*=0-09243 + 0-000215 T; 



wherein T is counted from 0°, and the tension corresponding to 

 a temperature of 35° is assumed to be 763*27 millims. It will 

 be seen by the following Table that the tensions which have been 

 computed from the above accord very well at low temperatures. 

 At high temperatures they show a somewhat greater disparity; 

 with the exception of a single case, however, it does not exceed 

 that which corresponds to a difference of 0°*1 of temperature. 

 In what manner the difference which appears at 120°, and which 

 corresponds to an error of o, 3 of observation, will render neces- 

 sary another determination of the value of a, can only be decided 

 by comparison with the original observations. A comparison of 

 the new Table with the earlier one communicated by Regnault, 

 only justifies the conclusion that there were difficulties to be 

 overcome in the experiments which prevented the determination 

 of the higher tensions of ether-vapour with the same accuracy as 

 those of lower temperatures. 



* Poggendorff 's Annalen, vol. xliii. p. 540. 



