4 



Prof. Buff on the Relation between the 



we can use it with the help of the now known values of u for the 

 calculation of the tensions of saturated vapours. This formula 

 differs from all others hitherto employed for the same purpose, 

 because it is deduced purely from theory, and because the num- 

 ber a, the only one occurring therein which is derived from ex- 

 periment, has a definite scientific signification. 



How far the numbers calculated by means of this formula 

 agree with experiment is perceived by comparison with, some 

 examples which have been taken indiscriminately from Regnault's 

 work, and which, with the numbers of the pages whereon they 

 occur, are contained in the following Table : — 





Temperature of the air- 











thermometer. 



Tension. 





Page in the 

 work. 











Difference i 



T°C. 



«. 



Calculated. 



Observed. 





599 



- 32 



0-05918 



0-25 



0-32 



-007 



509 



- 30-89 



005936 



0-34 



0-32 



4-0-02 





- 23-00 



006078 



0-70 



066 



+004 



599 



- 1600 



006201 



1-29 



1-29 



0-00 



509 



- 1030 



006301 



201 



1-99 



+002 





- 5-47 



0-06385 



2-99 



2-95 



+004 



508 



- 2-43 



006436 



3-76 



3-81 



-005 









006478 



4-50 



f 4-50 

 \ to 4-69 





504 



4- 2016 



0-06821 



1748 



17-64 



-016 



606 



4000 



007152 



54-90 



54-91 



-001 



526 



89-83 



007945 



522-00 



52202 



-002 





92-20 



007982 



57100 



569-83 



+ 117 



568 



116-30 



0-08346 



1324-21 



1324-62 



-0-41 





116-36 



008347 



1326-77 



1326-67 



+010 





151-37 



008856 



3711-88 



3708-96 



+2-92 





151-38 



0-08856 



3712-95 



3711*19 



+ 1-76 





160-28 \ 

 160-28/ 



0-08982 



4679-60 



/ 4677-43 

 \ 467904 



+2-17 

 +0-56 



570 



201-91 



0-09548 



1214510 



12147-94 



-2-84 





201-92 



0-09548 



1214807 



12155-63 



-7-56 



572 



230-50 



0099172 



21129 65 



2112700 



+266 





230-52 



0099175 



21136-89 



21136-72 



+0-17 





230-54 



0099177 



2114437 



21139-30 



+507 





230-56 



0-099180 



2115203 



21144 02 



+ 801 



The differences between the calculated and the observed ten- 

 sions have, it will be seen, sometimes a positive and sometimes a 

 negative sign, and in no case exceed the limits of unavoidable 

 errors of observation. Between 90° and 1 00° an error of y^th of a 

 degree in temperature makes one of more than 2 millimetres in 

 the tension, and at temperatures above 150° a difference of tem- 

 perature of only 2TT tn °f a degree corresponds to differences in 

 the tension of from 6 to 18 millimetres. 



As it has thus been proved that the values obtained from the 

 formula agree with very great accuracy with the results directly 



