NICHOLS. — THE VISIBLE RADIATION FROM CARBON. 81 



approximate accuracy of which must rest upon the fact that the melting- 

 points for palladium, platinum, etc., as given by Violle arc found to lie 

 upon what may reasonably be supposed to be an extension of the curves 

 experimentally determined for lower temperatures by means of the air 

 thermometer. As for the various formulae for the variation of electro- 

 motive forces of thermo-elemeuts with the temperature, we must not lo 

 si'dit of the fact that they are simply analytical expressions for experi- 

 mentally determined relations, and that the extension of them to temper- 

 atures lying far beyond the experimental range is not to be regarded as 

 more trustworthy than the extention of a curve by graphical methods. 



Under these circumstances I decided to content myself with the pro- 

 visional acceptance of the following values for the melting-points 

 gold, palladium, and platinum, namely : — 



Gold, 1075° C 



Palladium, 1500° C 



Platinum, 1775°C, 



and to ascertain as accurately as possible the electromotive force given 

 by the thermo-elements used at these points. It was thought that b\ 

 drawing a curve through them, and reading intermediate temperatures 

 from this' curve, the values obtained would be as close as our present 

 knowledge of the subject will admit. The platinum, platinum-rhodium 

 wire used for my elements was obtained, as has already been state I. 

 from Heraeus in Hanau and was supposed to be of the same stock as 

 that employed by Ilolboru and Wien. The fact that the electromotive 

 force given by these thermo-elements when exposed to the temperature 

 of melting platinum agreed very closely indeed with that obtained 1»\ 

 extrapolation of their data seems to indicate that the metals were identi- 

 cal with those used by them. 



Exhaustive studies at the hands of Le Chatelier,* of Barus,t and ol 

 Holborn and Wien. t and others have led to the conclusion that whenevei 

 thermo-elements consisting of platinum on the one hand, and ol the 

 alloys of that metal with iridium, rhodium, or any other metals of 

 platinum group on the other, are to be used in the measurements of 



* Le Chatelier, Comptes Rendus. CII. (1860) 819; Journal de IV 

 VI. 26 (1887) ; also Mesure des Temperatures fclevees [Paris, 1000), Chapter VI. 



t Barus, Bulletin of the U. S. Geological Survey No. 54 ; also American -lour 

 nal of Science, XLVIII. 336. 



t Holborn and Wien, Wiedemann's Annalen, XLVIT. 107 (1892); LV1 



560 (1895). 



VOL. XXXVII. — 6 



