RICHARDS. — TRANSITION TEMPERATURES OF SODIUM CHROiL\.TE. 181 



to Beckmann's well-known freezing point apparatus, consisting of two 

 large test-tubes, one within the other, immersed for nine-tenths of their 

 length in a roomy, transparent water bath. The inner test-tube, which 

 contained the mixed hydrate, was 3.3 cm. wide and 16 cm. long (in- 

 ternal measurements) and had a capacity of 120 cc. The outer tube 

 was large enough to allow a small air space between the two vessels at 

 every point except the top, where the inner vessel was insulated from 

 the outer one by a narrow rubber ring. This ring held the inner tube 

 in place, preventing heat conductance through the glass to the material, 

 and excluded the outer air from the space between the two tubes. 

 Thus all the heat obtained by the melting material came to it from the 

 outer bath through the confined air-jacket, which is an absolutely 

 necessary provision if accuracy is sought. 



The inner large test-tube containing the sodium chromate was closed 

 at the top by a rubber stopper, carefully freed from loosely adhering 

 matter which might contaminate the materials. It was pierced by two 

 holes, through which short glass tubes were passed. One of these tubes 

 was of the same length as the thickness of the stopper, and served to 

 hold the stirrer ; while the other reached 0. 7 cm. above the stopper and 

 below to within 1 cm. of the sodium chromate, and served to admit and 

 support the thermometer. To the upper end of the latter tube a short 

 length of stout rubber tubing was attached, the object of which was to 

 hold the thermometer in place at any desired level. A water jacket 

 around the thermometer is necessary only when the temperature of the 

 room is several degrees distant from 20° ; because a correction for the 

 protruding column can be applied with sufficient accuracy when the 

 temperature difference is small. 



The stirrer was made of a piece of platinum wire 15 cm. in length, at 

 one end bent into the shape of a ring, at the other end sealed into 

 a glass tube which served as a handle. 



The thermometers were three in number. The first, an excellent 

 Beckmann thermometer, the scale of which was divided into hundredths 

 of a degree, was used for the preliminary experiments, where the con- 

 stancy of the transition temperature of successive fractions was the main 

 issue. Two Baudin thermometers, 15200 and 15276, standardized with 

 the greatest care at the Bureau International des Poids et Mesures, 

 were used in the final determination of the points on the hydrogen 

 scale. These thermometers have been previously described ; ^^ their 

 scales are divided into tenths of a degree, and readings within (>.(»01° 

 were made by means of a Geneva cathetometer, observing the marks not 



" Richards and Wells, These Proceedings, 38, 43 (1902) ; Zeit. phys. 

 Chem., 43, 465 (1902). 



