94 PROCEEDINGS OF THE AMERICAN ACADEMY. 



and II., but the temperatures were higher throughout. With this junc- 

 tion it was found possible to penetrate to the centre of the flame without 

 the deposition of carbon, the temperature of the wire being apparently 

 too high to permit the formation of soot. Upon pushing through the 

 median plane of the flame to the second luminous mautle, the junction 

 was melted. This result was not unexpected, since the temperature of 

 the junction at the first luminous mantle reached 1750°, so that a rise of 

 twenty-five degrees of temperature would suffice to produce fusion. The 

 wire when pushed through the flame in the manner just described is 

 heated for greater and greater distances back from the junction until the 

 losses of heat at the junction are sufficiently diminished to raise the tips 

 of the wires to the melting-point. 



With Junction IV. (diameter 0.0082 cm.), a fourth curve, similar iu 

 form to the preceding ones and with still higher temperatures, was ob- 

 tained. This junction was fused at a distance of 0.075 cm. from the 

 core of the flame, and of 0.037 cm. from the edge of the first luminous 

 mantle. It was easy to observe in the enlarged image upon the plate of 

 the microcamera the melting away of the platinum wire, while the 

 platinum-rhodium alloy was still unaffected, and while contact was still 

 unbroken. A satisfactory observation of the electromotive force of the 

 thermoelement at the melting-point of platinum was thus obtained. This 

 reading (0.018236 volts) differs from the value found in my calibration 

 of the thermo-junctions used in this investigation (0.0182G2 volts) by a 

 quantity of (0.000026 volts) less than the errors due to changes in the 

 electromotive force of the standard cell. If the latter reading be taken 

 to correspond to 1775°, the former indicates 1773°. 



Beyond this point, it was impossible to make direct observations of 

 temperature ; but the form of this and the preceding curves were so 

 closely allied that I felt no hesitation in extending the curve d to the 

 core of the flame. This has been done by means of dotted lines in the 

 figure. Curves a and b have been extended in the same manner. In 

 order to form an estimate of the temperature which would have been 

 reached by a thermo- junction of negligible cross-section, provided such a 

 junction could have been obtained which was capable of registering tem- 

 peratures above that of the melting-point of platinum, the ordinates of the 

 four curves, a, b, c, and d were taken for the core of the flame, for the 

 plane of the luminous mantle, for a plane distant 0.07 cm. from the core, 

 and for a plane 0.10 cm. from the core. These readings were plotted 

 and curves were drawn through them as shown in Figure 12; relative 

 cross-sections of the wires being taken as abscissae, the temperatures as 



