and Rock Densities at High Temperatures. 9 



order not to interfere with the float. The e.m.f. of the ele- 

 ment was measured in terms of a standard cadmium cell with 

 the help of a Wolff potentiometer. The readings were con- 

 verted to degrees on the standard temperature scale, in the 

 manner described in recent publications from this laboratory.* 



The opening H (fig. 2) was kept closed by a plug of asbes- 

 tos, except for a few seconds while making the measurements, 

 in order to maintain the reducing atmosphere within. The 

 admission of a small amount of air at this point cannot be 

 avoided, however, and to prevent its oxidizing the lower cross- 

 bar of the cage, this was covered with a thin sheet of plati- 

 num. The loss of weight from the cage during a series was 

 then less than OI gram. This was corrected for in the calcu- 

 lations. 



The furnace required for this apparatus must give a uni- 

 form temperature up to 1600° in a space about 80 mm in diam- 

 eter and 200 mra long. For this purpose the carbon tube 

 furnace shown in fig. 2 was employed. It was built on the 

 same plan as our small tube-furnace used for the melting point 

 of platinum, f except that only the middle portion of the tube 

 was of carbon, ground on the lathe to a uniform thickness and 

 diameter. This was set into end pieces of graphite which fit 

 the brass water-cooled terminals and require no copper plat- 

 ing or soldering to give a good contact. The carbon tube was 

 surrounded by a fireclay tube, and the intervening space filled 

 with carborundum. The outside packing was of petroleum 

 coke. This furnace took about 750 amperes at 10-15 volts to 

 reach 1600°. 



After the earlier measurements the apparatus was enlarged 

 to obtain greater sensitiveness. The second furnace was of 

 the granular carbon resistance type, developed at the Koenig- 

 liche Porzellan-Manufaktur in Charlottenburg4 The heated 

 zone in this furnace was about 130 mm in diameter and 250 mm 

 long. 



The graphite apparatus was protected from oxidation by a 

 mixture of nitrogen and carbon monoxide, introduced through 

 a tube at the bottom. This gas mixture was made by passing- 

 air through a porcelain or iron tube filled with coke and 

 standing vertically in a nickel wire resistance furnace kept at 

 about 1100°. Only 18 grams of coke are required to furnish 

 100 liters of gas containing two parts nitrogen and one part 

 carbon monoxide. The gas was passed through suitable 



*Day and Sosman, this Journal, xxix, 98-161, 1910. R. B. Sosman, ibid., 

 xxx, 1-15, 1910. Dav and Sosman, Carnegie Institution Pub!., No. 157, 

 1911. 



f Sosman, this Journal, xxx, 1-15, 1910. 



t J. Bronn, Der Elektrische Ofen, p. 94, 1910. 



