316 PROCEEDINGS OF THE AMERICAN ACADEMY. 



effectiveness of the treatment is shown furthermore in the fact that 

 in over six months of continual use the inside has not stretched by so 

 much as an additional 1 / 10000". The hole was enlarged to a final size 

 of 17/32", instead of keeping it as small as possible, because of the 

 difficulty of reaming out the hole so as to give a satisfactorily smooth 

 surface after the seasoning process. The difficulty was occasioned 

 by the hardness of the steel, and several attempts were necessary 

 before the desired result was produced. 



The pressure measuring coil was the same as that used in the last 

 part of the work on ice VI. The construction of the insulating plug 

 was also the same as that used there. During the course of the work 

 it was necessary to take this plug apart several timess, because water 

 had reached the mica washers, and once or twice the mica washers 

 themselves have given way. These mica washers are the weakest 

 part of the entire apparatus as at present used, since they gradually 

 disintegrate and fail by shear after prolonged use, but it is a matter 

 of only a few hours to replace them. Every time after the insulating 

 plug has been freshly set up it has been tested for insulation resis- 

 tance, both during application of pressure and after release. The 

 resistance was in all cases as high as several hundred megohms, the 

 limit of the measuring devise. The steel of the insulating plug has 

 also failed once or twice by the "pinching-off effect" ^ after long use. 

 This also is an easy matter to repair. Failure of this type is attended 

 with some danger, however, because of the violence of the explosion 

 with which the ruptured plug is expelled. The surest way of avoiding 

 this danger is to so mount the apparatus that the plug points at the 

 floor or other indestructible object. 



The hydraulic press, the method of measuring the displacement of 

 the piston, and the details of the packing of the moving piston, were 

 the same as that used in the former paper. 



In the use of the apparatus to determine compressibility there is 

 one serious error which did not enter into its use in the determination 

 of the change of volume during change of state, namely the correction 

 for the distortion of the cylinder in which the piston moves. At low 

 pressure the correction is relatively imimportant, and may be com- 

 puted from the theory of elasticity, if one is willing to assume that 

 the theory is sufficiently accurate for this type of stress. But at higher 

 pressures the correction becomes more important, increasing in 

 percentage value directly with the pressure, and is almost certainly 



6 Rridgman, Phil. Mag., 24, 63-79 (1912). 



