A. L. Day and J. K. Clement — Gas Thermometer. 451 



are of the same order of magnitude and similarly distributed, 

 thereby confirming the conclusion that the observations are 

 more accurate and consistent, within this temperature region, 

 than the interpreting parabola. 



Metallic Melting Points. — By way of establishing perma- 

 nent records of these observations, the usual procedure was 

 adopted of determining various metallic melting points which 

 occur within the range of the temperatures investigated, with 

 the thermoelements which had been directly compared with 

 the gas thermometer. The metals chosen for this purpose 

 were gold, silver, copper and zinc. Metal melting points were 

 given the preference over pure salts which have been repeatedly 

 suggested for this purpose, (1) on account of the greater sharp- 

 ness of the melting point, (2) on account of their general 

 availability for such determinations, and (3) because of the now 

 very generally established custom of comparing the results of 

 different observers through the medium of these standard 

 melting points. 



In choosing the materials for such determinations, two not 

 altogether concordant standpoints must be recognized ; (1) the 

 materials used must be of absolutely known composition and 

 of high purity in order to give the melting point determina- 

 tions a positive significance ; (2) the same materials in the same 

 purity must be easily obtainable by other investigators in 

 order to enable the results to be conveniently utilized by 

 others if desired. The metals used in this investigation were 

 from various sources which will be specified below. Each has 

 been very carefully described and analyzed by Dr. E. T. Allen 

 of this laboratory, whose report is printed in full. We pre- 

 pared none of the metals ourselves. Those which were used 

 were purchased from firms who may fairly be expected to 

 supply the same nominal quality to any other investigator who 

 may care to use them, but it must be emphasized in this con- 

 nection that metals furnished under the same description by 

 the same dealer at different times have not always proved of 

 uniform purity and probably cannot at present be expected to 

 do so. The variations in the thermal behavior of the different 

 samples is not great, never amounting to more than 1° in our 

 experience ; but we are of course unable to offer any guarantee 

 that the same metals obtained in future will remain within 

 this limit, nor is the dealer's guarantee at present a sufficient 

 protection. 



As the situation now stands, the errors in the gas thermo- 

 meter measurements are rather smaller than the differences 

 between the melting points of different samples of a given metal 

 obtained at different times from the same dealer and of the 

 same (nominal) purity. This may serve to emphasize more than 



