122 A. L. Bay and R. B. Sosman — 



(2) Independence of particular experimental arrangements. 

 The melting point of a metal, for instance, must be sharp and 

 definite enough so that with different kinds of furnaces and 

 different rates of heating, the same temperature will be 

 obtained. 



(3) Convenience and safety of manipulation. A melting 

 point which can only be obtained by the use of elaborate 

 experimental arrangements is undesirable, even though it be 

 reproducible and sharp. Furthermore, the substance must not 

 injure the instrument to be calibrated. 



(1) Reproducibility. — No extensive experiments have been 

 made in the present work to test a large number of samples of 

 different origin. It appeared sufficient to assure ourselves 

 that all of the metals here used are obtainable in such degree 

 of purity, or with such a constant amount of impurity, that 

 the variations in their melting points are well within the limits- 

 of error in the scale itself. Waidner and Burgess* have 

 recently made comparisons of various samples of pure zinc, 

 antimony, and copper, and have found no differences exceed- 

 ing 0*3°. f Our experience has been the same. All of the 

 metals in the present investigation are readily obtainable from 

 the ordinary sources of supply. They have been carefully 

 analyzed in this laboratory by Dr. E. T. Allen, and the results 

 are given in section 6. 



(2) Independence of Experimental Conditions. — A number 

 of experiments were made to test the effect of different experi- 

 mental arrangements on the points. Two different furnaces 

 were tried, one 65 mm inside diameter and 150 mni long, the other 

 55 ffinl inside diameter and 230 mm long. The region of constant 

 temperature in the second furnace was longer than in the first 

 and accordingly there was a larger range in which the crucible 

 could be moved about without affecting the temperature. This 

 furnace was used for all work after March 6, 1909. The ulti- 

 mate test was always the agreement between the melting and 

 freezing points. Any serious disagreement of these two 

 shows that some influence is entering from without. 



The results of the study were briefly as follows : (1) The 

 best dimensions for a charge of metal are about 25 mm diameter 

 by 45 mm deep. (2) The thermoelement tube should be about 

 5 mm above the bottom of the crucible. (3) There is a region , 

 within the furnace in which the melting and freezing points 

 agree and are independent of the rate of heating or (within 

 limits) of the depth of immersion of the thermoelement : it is 

 necessary to find this position of the crucible by trial. With 

 this position once determined, the temperature of the zinc, 

 antimony, silver, gold, and copper points can be relied upon 



* Phys. Eev., xxviii, 467, 1909. Bull. Bur. Stds., vi, 149-230, 1909. 



fin the case of antimony, this statement applies only to Kahlbaum's metal. 



