BRIDGMAN. — A DETERMINATION OF COMPRESSIBILITIES. 275 



a possible error of ^ ^ in the area. The determination of the compres- 

 sibility of the steel, which must be made independently, takes the place 

 of the determination of the compressibility of the glass in previous work. 



Avariation of temperature of one degree is equivalent in displacement 

 of the piston to about 50 kgm. The pressure chamber in which the 

 cylinder was placed was inserted in a water bath as nearly as possible 

 at room temperature, and the small variations of this temperature 

 were read to 0.01° after every determination. The temperature at 

 the time of measuring the displacement, which was done with a read- 

 ing microscope, was also recorded and corrections applied for varia- 

 tions. The observations were carried out at temperatures varying 

 only slightly from 20°, and the final results are for this temperature. 

 The error from temperature variations, which were hardly as much 

 as 0.1°, becomes entirely negligible at the higher pressures, in which 

 the principal interest of this work lay. For accurate work at lower 

 pressures it would, of course, be necessary to take more elaborate 

 temperature precautions. 



Another correction necessary to apply is a correction on the measured 

 diameter of the piston, because the piston in advancing into the inner 

 cavity draws with it some of the molasses in the crack between piston 

 and cylinder. The effect of this is to increase the effective diameter 

 of the piston. The question has already been discussed in connection 

 with the absolute gauge and a method given for determining the cor- 

 rection, which, however, is not applicable here. In this case the cor- 

 rection was determined by first smearing the hole in the cylinder with 

 a heavy oil, inserting the piston, and then withdrawing it again. A 

 film of oil adheres to the piston equal approximately to one half of 

 the volume of the oil originally in the crack between piston and cylinder. 

 The quantity of oil thus clinging to the piston was determined by 

 weighing, and the crack in this manner found to be 0.0003 in. (0.00076 

 cm.) wide. The method of course is very inaccurate, but seemed the 

 only practical way of getting any idea of this small quantity. The 

 total correction thus introduced is only 1 per cent, so that fairly large 

 errors in the correction are unimportant. 



It seemed necessary to investigate one other source of possible error 

 before confidence could be placed in the results. There has been ex- 

 pressed a feeling that metals might be porous under high pressures, 

 the experience of Amagat in forcing mercury through 8 cm. of cast 

 steel being adduced as evidence on this point. To test this, a piece 

 of steel from the same piece as the piezometer was weighed before 

 and after subjection to pressure, in an endeavor to detect possible in- 

 crease of weight from the absorbed liquid. No change of weight of 



