360 Prof. W. Ramsay and N. Eumorfopoulos on the 



the power was varied by varying the potential- difference of 

 the carbons, and the alternating-current arc lamp used was 

 one which effected this variation automatically, even although 

 the power expended in the arc was varied from 200 to 600 

 watts. In order to complete the comparison of the continuous 

 and alternating-current arcs, it will be necessary to compare 

 the behaviour as regards illuminating-power of alternating- 

 current arcs, taking the same mean power but formed with 

 larger currents and less carbon potential -differences ; that is 

 to say, comparing alternating arcs of equal power-absorption, 

 but taking very different currents and therefore having dif- 

 ferent lengths. We hope to extend this investigation to 

 cover these additional questions at some future time. 



The above observations have necessitated an enormous 

 number of photometric and electrical measurements, and we 

 have in the above work been very efficiently aided by Messrs. 

 L. Birks, W. H. Grimsdale, A. M. Hanbury, E. K Griffiths, 

 and others, to whom our thanks are due. 



XXXY. On the Determination of High Temperatures with 

 the Meldometer. By William Ramsay, Ph.D., F.R.S., 



Professor of Chemistry, University College, London, and 

 N. Eumorfopoulos, B.Sc, Demonstrator of Physics, Uni- 

 versity College, London*. 



THE meldometer, an instrument devised by Dr. Joly, has 

 been sufficiently described by him (Froc. Roy. Irish 

 Acad. 3rd series, ii. p. 38, or Chem, News, vol. lxv.), and we 

 need therefore only give a very brief account of it here. The 

 essential part of the instrument is a length (about 10 centim.) 

 of thin uniform platinum ribbon, about 1 millim. wide. This 

 is heated by a current of adjustable strength, and the increase 

 in length of the ribbon is measured by a delicate micrometer- 

 screw, the ribbon being kept gently stretched by a small 

 spring. The temperature of the ribbon is, of course, lower 

 where the two forceps hold it ; but if it is suitably cut at 

 each end nearly to a point, a length of, say, 6 centim. in the 

 middle may be made of a very uniform temperature, as can be 

 proved experimentally by taking the reading of the melting- 

 point of the same substance at different points along the 

 ribbon. 



An infinitesimal quantity (scarcely visible with the naked 

 eye) of the substance to be melted is placed on the ribbon and 

 viewed with a low-power microscope. The small quantity of 

 * CommuDicated by the Physical Society : read February 14, 1896. 



