390 PROCEEDINGS OF THE AMERICAN ACADEMY. 



the equilibrium pressure with as little of the solid phase present as 

 possible. This evidently avoids the large depression of the fi-eezing 

 point which would result from the concentration of impurities in the 

 small remaining mass of liquid after the solid had nearly all separated 

 out. The precaution was probably supertluous because the mercury 

 was carefully purified to begin with, and no effect of such a nature was 

 ever detected. 



One other source of error was guarded against which was especially 

 prominent in the earlier work of 1 909. The fluid transmitting pres- 

 sure from the lower cylinder, in which was the absolute gauge, to the 

 upper cylinder, in which was the mercury, was a thick mixture of either 

 molasses and glycerine or glucose and glycerine. It was necessary to 

 use such a mixture in connection with the absolute gauge in order to 

 avoid leak at the high pressures. But there is the disadvantage that 

 the viscosity of either of these fluids increases so rapidly with .pressure 

 that high pressures are not transmitted immediately through the con- 

 necting pipe from the lower to the upper cylinder. This connecting pipe 

 was 15 inches long and 1/16 inch internal diameter. That the pressure 

 ■was transmitted slowly was shown by the fact that after pressure had 

 been increased in the lower cylinder by advancing the piston, the abso- 

 lute gauge in the lower cylinder indicated a fall of pressure for some 

 minutes, while the change of mercury resistance in the upper cylinder 

 indicated a corresponding rise, evidently due to the slow flow of liquid 

 from the lower to the upper cylinder. The effect was troublesome only 

 at the higher pressures, the pressure coefficient of the effect being very 

 high, and was very much greater for the mixture of glycerine and mo- 

 lasses used in 1909 than for the glycerine and glucose of 1911. In 

 the extreme case, glycerine and molasses at 12,000 kgm., it was neces- 

 sary to wait two hours for the equalization of pressure. For most of 

 the pressures used the effect disappeared more rapidly than the heat of 

 compression. To avoid error from this effect, the decrease of resist- 

 ance indicating freezing must come after an increase of pressure in the 

 lower cylinder, and the converse increase of resistance after a decrease 

 of pressure. The measurements were made Avith this in mind. 



The temperature measurements above zero were made wuth a Ton- 

 nelot thermometer calibrated at the Bureau of Weights and Measures 

 in Paris. Below zero the readings of 1911 were made w^ith a toluol 

 thermometer calibrated at the Reichsanstalt, and in 1909 with a mer- 

 cury thermometer made by Green of New York, the zero correction for 

 which was the only correction applied. In any case, the errors in the 

 temperature readings were less than the possible error in the pressure. 

 The temperature was kept constant with an ordinary thermostatic de- 



