NICHOLS. — THE VISIBLE RADIATION FROM CARBON. 81 



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

 points for palladium, platinum, etc., as given by Violle are 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 ot thermo-elements with the temperature, we must not lose 

 siofht 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 ihe 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 of 

 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 by 

 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 stated, 

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

 that employed by Holborn 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 by 

 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 of 

 Holborn and Wien,t and others have led to the conclusion that whenever 

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

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

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



* Le Chatelier, Comptes Kenrlus. CII. (\8Cf>) 819; .Innrnal de Physique (2) 

 VI. 26 (1887) ; also Mesure cles Temperatures Elevees (Paris, 1900), Chapter VI. 



t Barus, Bulletin of the U. S. Geological Survey No. -54; also Americ;in Jour- 

 nal of Science, XLVIIT. .336. 



t Holborn and "Wien, Wiedemann's Annalen, XLVIL 107 (1892); LVI 

 560 (1895). 



VOL. XXXTII. — 6 



