BRIDGMAN. — MERCURY UNDER PRESSURE. 361 



thermore, the friction of the packing inaterial, while not very great, 

 retarded greatly the quickness with which the piston moved in response 

 to an increase of pressure. The piston might continue to move for 

 some minutes in response to an increase of pressure on the low-pressure 

 side. The result was that the step-like advance of pressure with each 

 stroke on the low-pressure side was entirely wiped out and converted 

 into a continuous advance on the high-pressure side. The only way in 

 which a retrogression of pressure was likely to occur was by dissipation 

 of the heat of compression. If pressure is rapidly pushed to a maximum, 

 there is produced an increase of temperature. The final equilibrium 

 pressure is lower than the maximum, which corresponds to an unknowTi 

 temperature. The difficulty was avoided by compressing so slowly 

 that as the pressure approached the maximum the compression was 

 sensibly isothermal. With the apparatus so designed as to reduce the 

 volume of the transmitting fluid to a minimum, it was easy to attain 

 this condition. By comparing results with slower and more rapid com- 

 pressions a safe rate was found. The maximum pressure of 12,000, 

 for example, could be reached with entire safety in seven or eight 

 minutes. On releasing pressure there was the reverse effect due to 

 cooling, but it is readily seen that there is no error introduced here 

 unless there is actual retrogression of pressure, which is just as easy to 

 avoid as with increasing pressure. 



The temperature was kept constant during the compressibility de- 

 terminations by surrounding the pressure cylinder with a suitable bath, 

 at 0° with ice and water, and at 22° with an electrically regulated 

 thermostat. The piezometers were always submerged for some minutes 

 in this bath before the final adjustments were made. Thus if the com- 

 pressibility of water were being determined with the piezometer in the 

 upright position, the piezometer was submerged in a test tube of water 

 placed in the bath without the mercury in the upper cup. After tem- 

 perature equilibrium had been reached, the upper cup was carefully 

 dried with filter paper and the mercury introduced, the lower part still 

 being in the bath. If the piezometer was to be used in the inverted 

 position the procedure was more simple, merely waiting for tempera- 

 ture equilibrium while submerged in the upright position under water 

 or mercury as the case might be. The variations of temperature in the 

 bath at 22° were never more than a few hundredths of a degree. The 

 actual temperature was read with a standardized thermometer. 



The volume of the piezometers was found by weighing when full of 

 water at a known temperature and when empty. The filling was per- 

 formed in the manner above and was more satisfactory than filling 

 with mercury and weighing, since by the use of water it was possible 



