80 PROCEEDINGS OF THE AMERICAN ACADEMY. 



about 85°. The result of the calibration agreed, however, so well with 

 similar experiments made by placing thermo-j unctions of various sizes in 

 the non luminous outer envelopes of the acetylene flame, of the ordinary 

 gas flame, and of the flame of the candle, that I feel warranted in placing 

 much dependence upon them. 



The correction is not of the same size in the various cases, but the 

 differences are such as one would expect from the nature of the flames. 



This method of correcting for the loss of heat in a thermo-junction 

 was first employed by Waggener * in his investigation of the temperature 

 of the flame of the Bunsen burner. I became acquainted with his 

 research only after the completion of my experiments. 



Calibration of the Thermo-Elements. 



All our estimates of very high temperatures may be said to rest in 

 one way or another upon extrapolation. Tne upper limit of usefulness 

 of the air thermometer has been found to lie in the neighborhood of 

 1.300.° At this temperature Erhardt and Schertel, t in their admirable 

 but little known research upon the melting-points of alloys of silver, 

 gold, and platinum, were obliged to abandon direct determination ; and, 

 at about the same temperature, Holborn and Wien and Holborn and 

 Day I in their latest studies upon thermo-electric thermometry found that 

 the indications of the air thermometer, even when constructed of the 

 most refractory of modern porcelain, began to be erratic. We have, it 

 is true, the investigations of Violle § upon the melting-points of the 

 metals of the platinum group; but these, it must not be forgotten, are 

 based upon an assumed value for the specific heat, and this assumption is 

 equivalent to the extrapolation of the curve of the variation of the 

 specific heat with temperature. The observed values, by means of 

 which this value was determined, all lie far below those of the melting- 

 points of the metals in question. It is necessary, therefore, in spite of 

 the accumulation of indirect evidence of their approximate accuracy, to 

 hold in reserve the assignment of absolute values of these melting-points 

 until by some means as yet unthought of we shall be able to obtain 

 direct experimental data. In the meantime, they afford us the best 

 present available basis for a temporary scale, our confidence in the 



* Waggener, Wiedemann's Annalen, LVIII. 579 (1896). 



t Erhardt and Schertel, Jahrbuch fur das Hiittenwesen in Sachsen, 1879, p. 

 154. 



\ Holborn and Day, American Journal of Science, VIII. 1G5 (1899). 



§ Violle, Comptes Rend us, LXXXIX 702, 1879. 



