104 REPORTS OF INVESTIGATIONS AND PROJECTS. 



nace in a gas-tight bomb in which the pressure outside the bulb can be main- 

 tained equal to that within for all temperatures. This offers three distinct 

 advantages : It provides against the deformation of the bulb through differ- 

 ences of pressure within and without in the region where the bulb material 

 becomes softer. By using the same gas within and without, there is no tend- 

 encv to diffuse through the bulb-wall from within or without. It enables the 

 initial pressure to be varied within considerable limits, thereby increasing both 

 the scope and sensitiveness of the manometer. The sensitiveness in our in- 

 strument with this arrangement was about three times that of the Reichsan- 

 stalt. (3) The expansion of the bulb material was determined with great care 

 and is probably accurate within 0.5 per cent. (4) The unheated space be- 

 tween the bulb and manometer has been reduced until the total correction in 

 this hitherto uncertain region amounts to less than 5° at 1,100°. An error of 

 5 per cent in the determination of its volume or temperature distribution is 

 therefore practically negligible. It is probable that these changes serve to 

 reduce the aggregate error of the gas thermometer in the region of 1,100° to 

 about one-tenth the magnitude which existed at the time of the establishment 

 of the present scale. 



Furthermore, and most important of all, these refinements are not limited 

 to this temperature region. It is therefore reasonably probable that the gas 

 scale can be extended to 1,500° or 1,600° with proportionate accuracy. The 

 immediate future of the present investigation will be to undertake this ex- 

 tension. 



The interpretation of these measurements in terms of the melting-points of 

 readily available substances encounters certain difficulties. The melting-point 

 of pure salts is not sufficiently sharp and is somewhat difficult of interpreta- 

 tion. The metals which have commonly been used for the purpose are not 

 obtainable commercially in sufficiently uniform purity to guarantee an accu- 

 racy of more than 1° at the higher temperatures. This is too large an error 

 for the interpretation of the gas-thermometer scale in its present refinement. 



No effort has been made to prepare metals in our own laboratory of excep- 

 tional purity, for the reason that such metals would not be available for 

 general use and would therefore be of little service. We have accordingly 

 adopted metals which are carried permanently in stock by dealers from whom 

 the same metal in a nominal quality equal to that which we used can be read- 

 ily obtained. We have analyzed these with extreme care to show the exact 

 content of the sample supplied to us. We have duplicated the purchases 

 ourselves, and have found no errors greater than 1° in melting-point deter- 

 minations. 



Another difficulty arises from the fact that the melting-points of the purest 

 metals available for use as constants in reproducing a high temperature scale 

 (zinc, silver, gold, and copper) are distributed in such a way that, although 

 they may be located upon the gas-thermometer scale with a probable error 

 not greater than 0.5°, the calculation of a similar curve passing through these 

 points docs not suffice to reproduce the scale with this accuracy. In the 

 region midway between zinc (418.9°) and silver (958.5°) the error of inter- 

 polation may amount to 2°, even with metals of exceptional purity. Extra- 

 polation is even more uncertain. This can be avoided by locating inter- 

 mediate points which are equally trustworthy, if such can be found. We 

 have not been fortunate enough to find points which fulfill these conditions 

 satisfactorily, but hope that we may yet be able to do so. 



