414 PROCEEDINGS OF THE AMERICAN ACADEMY. 



tion in the frame of the press, the cylinder at the high pressure end 

 taking up in itself the effects of the high pressure. The effect of dis- 

 tortion in the frame was avoided, therefore, by adjusting the low pres- 

 sure before every measurement of displacement to the same constant 

 value for all values of the high pressure in the immediate neighborhood 

 of the freezing point. 



The Pressure Measurements. — The method of measuring these high 

 pressures, with a discussion of the various sources of error has been 

 given in another paper. Briefly, the pressures were obtained from the 

 measured change of resistance of a coil of manganin wire immersed 

 directly in the kerosene of the pressure chamber which contained the 

 mercury under investigation. The coils, four of which were used dur- 

 ing this investigation, were each calibrated separately against an ab- 

 solute gauge. These were capable of rejjroducing the gauge with as 

 much accuracy as the gauge itself could be read, which was about 8 

 kgm./cm.^. 



The Data. 



The actual determinations by this method extended over about two 

 months, from Oct. 13 to Dec. 3, 1910. In all, eleven points were de- 

 termined, from —20° to +18°, both with increasing and decreasing 

 pressure. The measurements were sandwiched in between similar 

 ones on water which are to be described in another paper. The ap- 

 paratus was taken apart and set up a great number of times, both in 

 the course of the ordinary manipulations attending the experiment 

 and in making those changes necessary because of the piecemeal ex- 

 plosion of various parts of the apparatus. There were at least five 

 such explosions during the experiment ; two lower cylinders being 

 burst, two ujjper cylinders, and one connecting pipe. The accuracy 

 of the work is spoken for by the fact that determinations with all these 

 different groupings of apparatus, including change of the pressure 

 measuring coils, gave points lying on the same curve. This does not 

 appear so strikingly from the actual data given, but it must be re- 

 membered that by far the greater number of measurements during this 

 time were made on water, where the same independence of the par- 

 ticular piece of apparatus is also shown. The first five measurements 

 on mercury were made with different combinations of apparatus ; the 

 last six were made with the same set up. The equilibrium pressures 

 found in all eleven sets of readings are equally worthy of acceptance, 

 but the volume measurements of the first five must be discarded be- 

 cause the effect of changes of room temperature was not sufficiently 

 recognized at the time, and no correction was applied for it. The 



