388 PROCEEDINGS OF THE AMERICAN ACADEMY. 



be redetermined. The measurements were made by weighing mercury 

 under CS2, and plotting weight against temperature. Melting is ac- 

 companied by a sudden decrease in the displacement and so by a 

 sudden increase in the apparent weight. 



The Freezing Curve by Change of Resistance. 



The points on this curve were obtained on two separate occasions ; 

 the fall of 1909, and January, 1911, and with two different pieces of 

 apparatus. The data of 1909 are not as accurate as those of 1911, be- 

 cause the gauge was destroyed by an explosion before an entirely satis- 

 factory calibration was made. 



The details of the apparatus have been described elsewhere. The 

 absolute gauge, against which the pressure measurements were made 

 directly, has been described in a separate paper. The details of the 

 glass capillary containing the mercury resistance, and the electrical 

 connections with the details of the insulating plug have been described 

 in the paper on the use of mercury as a standard of pressure. The 

 pressure range employed there was only 6800 kgm./cm.^ ; it has since 

 been found that the same arrangement of apparatus is good to at least 

 12,000 kgm./cm.^, the range of this paper. The measurements of elec- 

 trical resistance were made by a null method on a Carey Foster bridge, 

 exactly as in the preceding paper. 



One slight change in the details of the measurements as made in 

 1911 should perhaps be noted, the employment of kerosene as the 

 liquid surrounding the glass capillary instead of a mixture of glycerine 

 and water. This avoids all possibility of short circuiting the terminals 

 of the plug dipping into the mercury cups, and also avoids the neces- 

 sity for carefully protecting these terminals. A light oil like kerosene 

 is necessary since a heavy oil, such as ordinary lubricating oil, freezes 

 at about 4000 kgm. at room temperature. It was found by inde- 

 pendent experiment that at least up to 12,000 kgm. the kerosene 

 does not become so stiff as to refuse to transmit the pressure 

 hydrostatically. 



The experimental procedure was the one which would naturally sug- 

 gest itself, but there are a number of details of manipulation which 

 were necessary to observe. The resistance was measured at appropriate 

 intervals of pressure, the temperature being kept constant. After every 

 increase of pressure it was necessary to wait 10 or 20 minutes for the 

 dissipation of the heat of compression to the surrounding bath, which 

 was kept constant to within O^.Ol with a thermostat. With increasing 

 pressure it was never found possible to reach exactly the freezing pres- 



