On Sur fusion in Metals and Alloys. 449 



FIG. 2. 



before solidifying. The third type of curve, which may be a modi- 

 fication of the other two types, indicates the occurrence of surfusion, 

 and, as a case in point, pure gold has been taken, the bend at a, 

 fig. 3, showing the amount of surfusion which was observed. I 

 have detected pronounced cases of surfusion not only in gold but in 

 copper, bismuth, antimony, lead, and tin. Surfusion, moreover, is 

 not confined to pure metals, and I showed in 1893* that the eutectic 

 alloy in the bismuth-copper series presents a marked case of sur- 

 fusion. 



FIG. 3. 



In order to study surfusion, it is necessary to make the galvano- 

 meter, to which the thermo- junction is attached, very sensitive, and, 

 by suitable adjustment, it is easy to catch on the sensitised plate any 

 desired portion of the range of the spot of light. It is, however, 

 preferable to balance by a potentiometer the current which results 

 from the heating of the thermo-junction, and in this way to prevent 

 the mirror from swinging through a long range. The sensitiveness 

 of the instrument is but slightly diminished by the introduction 

 of the potentiometer. As the thernaya- junction cools down, the spot 

 of light from the galvanometer is simply made to traverse a short 

 distance many times, instead of a long range once. The sensitised 



* "Second Report, Alloys Research Committee," 'Mech. Eng.,' 1893, Plate 32. 



