Nitrogen Thermometer from Zinc to Palladium. 123 



within 0*2°. With large charges and facilities for stirring the 

 metal, Waidner and Burgess have found the zinc point to be 

 reproducible in a given furnace, with a given sample, within 

 less than 0\L°. 



White* showed that the temperatures of the two silicate 

 points used for the present scale are reproducible within 1*0° 

 independently of the dimensions of the furnace or the rate of 

 heating. For a mineral melting point, the charge should be 

 small (about 3 grams), the heat should approach the thermal 

 junction from the side and not from the ends, and a position 

 in the furnace should be found in which the melting point, 

 determined by a bare thermoelement, does not vary with the 

 rate of heating. 



The possibility has been several times suggested that the 

 temperature of the thermoelement inside of the tube might 

 possibly be lower by a small constant amount than the tem- 

 perature of the metal outside of the tube, and that this error 

 might not be brought to light by such experiments as have 

 been described. Several melting and freezing points of cop- 

 per were, therefore, determined by enclosing the entire thermo- 

 element wire in a thin capillary of silica glass which was 

 slipped over the wire, bent double, and melted down upon the 

 wire at the junction by heating in the oxyhydrogen flame. 

 This was dipped directly into the molten copper to within 5 mm 

 of the bottom, so that there was practically no possibility that 

 the temperature of the junction could be lowered by radiation 

 or conduction upward. The melting point on element D 

 obtained in this way was 10,473 microvolts as compared with 

 10,473 microvolts in the closed glazed tube. There appears to 

 be no error from this cause. 



Convenience and Safety of Manipulation. — Zinc and gold 

 are the most convenient of manipulation, as they require no 

 special atmosphere and the temperatures are easily reached. 

 Antimony, silver, and copper require an atmosphere of car- 

 bon monoxide and are somewhat less convenient. More care 

 needs to be taken with copper than with silver and antimony 

 because of the considerable effect of a very small amount of 

 oxide. Antimony, silver, gold, and copper were all melted in 

 carbon monoxide, made by dropping formic acid into warm 

 sulphuric acid, and purified by passage through sodium 

 "hydroxide, lead nitrate, and sulphuric acid. The lead nitrate 

 was introduced to make certain that no trace of hydrogen sul- 

 phide, which might be formed if the acid became too dilute 

 or too warm, could pass into the metal. 



The two silicates (diopside and anorthite) and palladium were 

 melted in air. The silicate points are very convenient to 



* Diopside and its Belations to Calcium and Magnesium Metasilicates, this 

 Journal (4), xxvii, p. 5, 1909. 



