418 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



which the calibration of the resistance was made has a limit of sensi- 

 tiveness of about 8 kgm./cm.^, while the resistance measurements are 

 sensitive to about 2 kgm. The scale of the graphical construction 

 was so enlarged that 1 cm. on the bridge wire goes into 5 cm. on the 

 diagram, and 0.01 inch piston displacement into 1 cm. on the diagram. 



TABLE XI. 



Change of Volume of Mercury on Freezing as a Function of the 



Freezing Temperature. 



Ordinarily four points were found on the curve both above and below 

 the transition point, covering a range of from 500 to 700 kgm. in both 

 directions. Over this range, and within the errors of reading, the 

 curve connecting piston displacement with the position of the slider 

 on the bridge wire was almost without exception linear, and was 

 drawn in with a ruler. There was no tendency whatever for either 

 corner of the melting curve to be rounded off, indicating that the 

 change of state takes place at a single definite temperature and pres- 

 sure, and affording the best possible evidence of the sufficient purity 

 of the mercury. To this fact of absolutely sharp freezing is to be 

 attributed the self-consistency of the results. With water, where there 

 is some slight rounding off of the corners, the results are not so self- 

 consistent, although the total change of volume is larger. 



The process of correction applied to the observed values is shown in 

 the table in two steps so as to give an idea of the magnitude of the 

 corrections. In column (1) the corrections for the thermal dilatation 

 of the kerosene on passing from the lower to the upper cylinder and 



