Coefficient of Expansion of Incandescent Platinum. 367 
TABLE IT. 
~ Siemens. : Pt. 
Length. a —— ————= |} Benoit. | Matth. |thermom. 
a b d e f 
10000 1°000 0° 0° 0° 0° 0° 
1:0032 27000 325° 402° 420 378° 342° 376° 
1:0082 3°000 2 812 1108 708 72 i 
10146 4000 1086 1244 1950 1000 1170 1623 
10280 5000 1464 1682 3170 1272 1638 3100 
Cae 6000 || 1828 2072 oe 1512 2158 cee 
ue 7000 |} 2170 2387 exe 1766 2800 ere 
sarees 8-000 | 2470 | 2692 rae 1978 ar Sil pet ig 
| Bass 
__A glance at the curves and at this table suffices to show how 
ill-deserved is the confidence generally felt in these formule. 
The discrepancies involve differences of hundreds of degrees. 
IV. The methods employed by Dr. Siemens in the measure- 
ments represented by curves 4 and ¢ were identical; but the 
platinum used contained slight impurities. T'o these impurities 
the disparity was due. Dr. Siemens found that such foreign 
substances as usually occur in commercial platinum affected 
th the resistance of the cold metal and the law of the change 
of resistance with the temperature. 
Benoit’s formula (d) depends for its accuracy upon the 
determination of the boiling points of mercury, sulphar, cad- 
mlum and zine; for which Vol Deraraye® he adopted the values 
given by Deville and Troost.* M. Ed. Becquerel opposed 
those values at the time of their publication, and later researches 
have confirmed him, at least so far as cadmium and zinc are 
concerned, in thinking them to be entirely too high. 
In the following table the results obtained by Deville and 
Troost are compared with the more probable values given b 
other physicists. 
TABLE III. 
Boiling points. | Boiling points. 
Metals. 
Dey. and T. Other values, 
Hg. 360° 350° Regnault. 
8 4 448 ennett, This Journal, 1878. 
- } 446) § Carnelly and Quart. Jour. Chem. Soc., 
Cd. 860 712 { 1 |\Williams, 1876-78. 
Zn, 1040 8 Becquerel, Comptes Rendus, 57. . 
_ The substitution of these values in Benoit’s formula, places 
it more at variance than before with the measurements of 
Matthiesen and Siemens; a variation probably due to the 
* Deville and Troost, Annales de Chimie, III, vol. lviii. 
