THERMO- ELECTRIC PROPERTIES OF CHARCOAL AND CERTAIN ALLOYS. 335 
As before a 
= ==" 250003 
and thus the PtIr,, line is determined. It will be found on Plate XXI. 
Its equation is 
y= +:0199¢+ 70-21 . 
The platinum line (according to Tait) is laid down on the diagram: the 
iridium line is not yet determined, but at low temperatures it is, according to 
SEEBECK* and Marruiessent not far from the platinum line. Thus, the lines 
of none of these alloys are between those of their constituent metals. So far as 
the above four are concerned, at low temperatures the greater the percentage 
of iridium the higher is the line on the diagram. For temperatures above 
about 100° C., however, the order in the thermo-electromotive series is 
oir, Ptir,, Ptir,, Pt,{ Ptlr, When we consider the position of the 
limes of alloys of platinum and iridium, which have been determined by 
Professor Tart, the above simple relation between the constitution of the alloy 
and position on the diagram does not seem to hold even for low tempera- 
tures. Comparison of our diagram with his shows the order in the thermo- 
electromotive series at about 10° C. to be: (+) PtIry, PtIr,,, PtIry, 
ee Ptir;t PtIr,,{ Ptir,,{ PtiIrj, Ptlr, At 300° C the order is: 
Semetic,, Ptir,, Ptlir,, Ptir,,{ Ptlr;,¢ Ptlr,,t PtIr,,{ Pt, PtIr, (—). 
The difference in the position of lines of alloys of the same composition 
can be due only to differences of molecular state, or to slight impurities. 
These results do not warrant the conclusion to the relation between the 
constitution of the alloys and their order in the series which ROLLMANN 
and others have found to hold in the case of tin-bismuth and other 
alloys.§ Possibly, however, high temperatures would so change the series 
as to give this relationship only a narrow validity. It is interesting to 
note that the coefficient of proportionality of the specific heat of electricity to 
absolute temperature for platinum-iridium alloys is in some cases zero, in some 
| greater, in others less than zero. 
Iron-Gold Alloy, containing 5 per cent. of Tron. 
This alloy was examined by itself. The N wire in both circuits was attached 
to the same end of the galvanometer wire. The temperature of the beakers 
Was 14°6° C. For a short time it sank to 144°. The observations are given 
in Table IX. 
'|* “Pogg, Ann.” Bd. vi. pp. 133, 253. t See Professor Tart’s paper cited above, pp. 138, 139. 
+ “Pogg. Ann.” Bd. ciii. p. 112. § Wrepemann’s “ Galvanismus,” Bd. i. § 594, p. 814. 
