432 PROCEEDINGS OP THE AMERICAN ACADEMY. 



The general plan of the apparatus used in this work was similar 

 to that described in the j^revious papers, but there were some modifi- 

 cations in detail. The essential parts of the arrangement are shown 

 in the accompanying figure. 



The sodic sulphate was placed in a large test tube in a Beckman 

 freezing-point apparatus. It was found absolutely necessary to surround 

 this test tube by an air-jacket in order to prevent a too sudden inflow of 

 heat, which results in the rapid transition of the hydrated salt and conse- 

 quent superheating. The outside bath was easily maintained about half 

 a degree above the transition temperature by means of a small platinum 

 resistance coil in the water. The galvanic current used to supply this 

 energy was regulated by an external rheostat. With this arrangement 

 the transition temperature was easily maintained many hours at a 

 time. 



As is well known, the true reading of a mercury thermometer is 

 obtained only when the whole stem is the same temperature as the bulb. 

 It was evidently mcouvenient to surround the whole stem with sodic 

 sulphate, so the following device was employed. A long tube, of clear 

 glass and even bore, of a diameter about three times as great as the 

 thermometer, was used as a jacket to enclose the thermometer. Through 

 the jacket was kept running a stream of pure water, maintained at a 

 definite temperature by a large thermostat. The water, which entered 

 through a long thin tube parallel with the thermometer, rose through 

 the jacket and flowed out at a side neck at the top. The thermometer 

 bulb emerged from the jacket below through a rubber connection, which 

 was carefully wired with fine platinum wire. In use the jacket almost 

 touched the surface of the sodic sulphate. The diagram facilitates the 

 understanding of the arrangement. 



Since the cubic dilation of mercury is 0.000182 and that of hard glass 

 is about 0.000022, the apparent dilation of mercury in glass is about 

 0.00016. A projecting column thirty-six degrees in length, subjected 

 to a change in temperature of 0°.2 C, will therefore record a change of 

 0°.001 C. in its reading. Since, if possible, we did not wish the error to 

 exceed this amount, the thermostat was not allowed to vary in constancy 

 more than 0°.2 C, and the thermometer used to record its temperature 

 was itself standardized with sodic sulphate. It should be remarked that 

 in most cases an auxiliary thermometer would be a sufficient guide to the 

 correction for the projecting column since this great degree of accuracy 

 is not often sought. The jacket was used in this particular case in 

 order to make certain of the temperature of the stem throughout its 



