Calometric Absolute Measurements. 137 



and Joule's result becomes 429*9 m.-k. Unfortunately, in 

 this measurement also, Joule took for the basis the degree of 

 the mercury thermometer, and not that of the air thermometer, 

 and thereby made a precise comparison of his final result' with 

 ours impossible. Thus much may, however, be regarded as 

 established, that so soon as Joule's mercury thermometer does 

 not differ very considerably from the air thermometer, a tole- 

 rably good accordance exists between the results of the mea- 

 surements made by Dr. Joule and by myself. 



Von Quintus Icilius did not gauge the resistances made use of 

 in his numerous measurements* according to absolute measure. 

 The absolute resistance-values which formed the basis of his 

 calculations he ascertained by a comparison of his resistances 

 with the second copy of Jacobi's resistance-unit, produced by 

 Wilhelm Weber for himself, and gauged by him according 

 to its absolute value. This copy of Jacobi's standard was 

 = 0*9839 of Jacobi's unit; and since, according to W. Weber's 

 absolute-resistance measurements, the absolute value of Jacobi's 



resistance-unit is = 0*5 9 8 x 10 10 ( — - ), the copy had the ab- 

 solute value 0*5884 x 10 10 ( '). Von Quintus Icilius 



\ sec. / 



regards as the most trustworthy of his experiments the 34 in 

 which water was employed as the calometric liquid. From 

 these 34 experiments he calculates, as the final result, J = 3997 

 m.-k. Singularly, this result has not in the least aroused the 

 attention of physicists ; and yet it was to be inferred there- 

 from, either that the measurements which conducted to it were 

 very faulty, or that the theoretical views which formed its 

 basis needed correction. The essentially different result ob- 

 tained by me, in which by repeated trials I could detect no error, 

 and the good accordance of which with Joule's results I could 

 not but consider a further sign of its approximate correctness, 

 caused me to reflect long upon the cause of the discrepancy ; 

 at last I succeeded in attaining a complete explanation: — 

 W. Weber, in his first absolute-resistance determination, found 

 for the absolute value of Jacobi's resistance-unit about 8 per 

 cent, too small a number, in consequence of which Quintus 

 Icilius's final result could not but come out just as much too 

 little. If this error be corrected, the latter value (399*7 m.-k.) 

 becomes 431*6 m.-k., a value which, certainly, is somewhat 

 greater than that which results from Joule's measurements 

 and my own experiments ; but taking into consideration that 

 Quintus Icilius has quite neglected the variation of the hori- 



* Pogg. Ann. vol. ci. p. 65. 



