474 Prof. Silas W. Holman on 



The exponential equation applied to the Barus data yields 



2J £=0-7691 T r373 -1730, or 



^ = 0-7691 r 1 ' 373 , and £=1730 mv. 

 Range of data 350° to 1075° C. 



[N.B. This equation was deduced with the value 0°C.= 

 273°*7 absolute, whereas in all subsequent tables 0°C. = 273 o, 

 absolute is employed as a more probable value. The numerical 

 values of the constants are therefore subject to a slight modi- 

 fication, but as for the present purpose we are concerned 

 only with 3, which would not be sensibly changed, the 

 recomputation is not worth while.] 



Columns 6 and 7, Table II., give 8 and its percentage value 

 for the exponential equation. 



The Barus Equation. — The excessive labour involved in 

 the evaluation of the constants P, Q, P', and Q' of Barus's 

 proposed equation detracts so seriously from its usefulness 

 that I have also allowed it to deter me from computing them 

 for the above tabulated values. The comparison of the values 

 of 8 for his " equation 3," and for an approximate exponential 

 of my own based on the same data, is, however, decidedly in 

 favour of the latter. 



The logarithmic equation applied to the Barus data yields 



2j«=2-665 t V22 \ 

 or its equivalent, 



log So* = 1-220 log t + 0-42570. 



The deviations are given in the last two columns of Table II. 

 Holborn and Wien Data. — This important comparison * of 

 the rhodo-platinum thermo-couple with the porcelain bulb 

 air thermometer up to high temperatures was performed 

 under the auspices of the Beichsanstalt at Berlin, and appears 

 to be on the whole the most important and reliable contribu- 

 tion to this subject in recent years. The experimental work 

 was evidently conducted with great care, and although not 

 showing the concordance of results, nor the multiplication 

 of observations of Barus's work, yet in respect to stem- 

 exposure correction, to exposure of the thermal junction, 

 and to direct measurement of the coefficient of expansion of 

 the bulb, it is probably more free from systematic error. It 

 is to be regretted that the results were not more thoroughly 

 discussed, and that neither a chemical analysis nor even a 



* Holborn and Wien., Zeit.f. Instk. xii. pp. 257, 296 (1892) ; also in 

 full in Wied. Ann. xlvii. p. 107 (1892). 



