NICHOLS. THE VISIBLE RADIATION FROM CARBON. 85 



will be clearly seen upon the ground-glass screen, the ends of the loop 

 of metal projecting towards the flame. 



The determination of the electromotive force corresponding to the 

 melting-point is made as follows. The observer seats himself in a 

 position where he can watch closely the image of the flame and of the 

 thermo-element and moves the former gradually toward the junction, 

 balancing the potentiometer approximately from time to time as the 

 electromotive force rises with the increasing temperature. 



At a definite distance from the luminous envelope of the flame, which 

 distance depends upon the character of the metal under investigation, 

 the projecting ends of the loop will be seen to melt. So quiet is the 

 flame, and so well fixed the temperature gradient from its surface out- 

 ward when a proper burner is used, and when the flame is placed in a 

 locality reasonably free from air currents, that the fusion of the succes- 

 sive portions of the metal loop may be brought about from the end in- 

 ward with the greatest nicety; and the electromotive force may be 

 determined at each stage until the fusion has progressed to the plane 

 coinciding with the face of the junction. Even then, in many cases, 

 those portions of the loop of metal which lie within the angle of the 

 junction will remain unfused, although their distance from the melted 

 portion of the loop is only a fraction of a millimeter. 



The delicacy of this operation under favorable conditions is very great, 

 and the agreement of the successive readings of the melting-points of a 

 fiven sample of metal is excellent. It is desirable to make a series of 

 readings, leading up to the true melting-point, for the reason that when 

 the fusion of the metal loop has progressed to that portion which lies in 

 contact with the platinum, an alloy is almost immediately formed bet urn 

 the fused metal and the junction itself, which affects the thermo-electric 

 indications of the couple. For this reason it is not possible to get con- 

 sistent readings by repeating observations with a given junction. The 

 proper procedure is to cut the wires back 2 or 3 mm. from the apex of 

 the V after each set of readings, ami to make a new junction of tin; proper 

 form from the free ends thus produced. This requires but little time 

 after the operator has gained a reasonable degree of familiarity with the 

 method. 



When the metal, the melting-point of which is desired, is platinum 

 itself, the platinum wire of the junction begins to fuse at the same time 

 as the loop, the platinum rhodium or platinum-iridium side remaining 

 unmelted. The precise point at which this fusion of the platinum occurs 

 is, however, quite as definite as in the case of metals of lower meltiDg 



