Nitrogen Thermometer from Zinc to Palladium. 121 



elements at those temperatures can be obtained by algebraic 

 addition. This method offers a great advantage in that the 

 temperature need be only approximately constant and approxi- 

 mately known, since the differences in most cases amount to 

 only a few microvolts. By this method the comparison can be 

 very quickly made at 1500° in the blast-lamp flame, which, 

 with a little care, can be made to give a temperature constant 

 to 20°. 



D. Fixed Points. — Considerable attention was given in the 

 previous paper to the standard melting points which serve to 

 establish the gas thermometer scale for general use. In parti- 

 cular, a study was made of the purity of the zinc, silver, gold, 

 and copper used, and of the magnitude of the errors likely to 

 arise with the ordinary metals obtainable in the market.* 

 During the present work, attention has been more particularly 

 directed to the technic of melting point determination itself, f 



All the metal melting points here described, except that of pal- 

 ladium, were made in an upright cylindrical furnace through 

 which passed a glazed porcelain tube which could be tightly 

 closed above and below and therefore permitted the atmosphere 

 about the melting metal to be perfectly controlled. An effort 

 was first made to accomplish this by placing the entire furnace 

 inside a gas-tight bomb in which the atmosphere could be 

 similarly varied, but the persistent retention of gases by the 

 various clay insulating materials used about the furnace made 

 this method slow, cumbersome, and very uncertain in its results. 

 The only success which these bomb furnaces attained was to 

 permit melting points to be measured in an approximate vacuum 

 (about l mm pressure). But it has since been found so much 

 simpler to operate with a neutral or reducing atmosphere in 

 the closed tube passing through the heated zone, that the 

 vacuum furnace has not been used for this work. 



The chief disadvantage in the use of a tube of this kind is 

 its effect upon the temperature gradient along the furnace axis. 

 More heat is diverted toward the ends of the furnace and the 

 central constant temperature zone becomes shorter. It offers 

 no difficulty except that greater care must be taken in locating 

 the crucible within the constant temperature region. 



The qualities desired in fixed thermometric points for estab- 

 lishing and reproducing a scale are : 



(1) Exact reproducibility of the temperature in repeated 

 determinations with the same charge of material and with a 

 different charge independently obtained. This means that 

 the metal or salt must be either perfectly pure or obtainable 

 with a constant amount and kind of impurity. 



*E. T. Allen, in paper of Day and Clement, loc. cit., p. 454. 

 f See also W. P. White, Melting Point Determination and Melting Point 

 Methods, this Journal (4), xxviii, 453 and 474, 1909. 



