BRIDGMAN. — WATER UNDER PRESSURE. 449 



two dififerences, from which the efiFect of the " second part of the vol- 

 ume " has been eliminated. These two differences involve the change 

 of volume with temperature at constant pressure of the material oc- 

 cupying the same volume that the bessemer steel would have under 

 the same conditions of temperature and pressure. The quantities en- 

 tering into the two differences are, therefore, the thermal dilatation of 

 bessemer steel, of gasolene, and of water at various pressures. Now the 

 thermal dilatation of bessemer steel is very small comparatively, and 

 is furthermore known to be little affected by changes of pressure. For 

 the accuracy required in this work, therefore, the change Of volume of 

 the steel cylinder, both with temperature and pressure, may be assumed 

 to be known. This leaves only two unknown quantities in the two 

 differences mentioned above, so that either may be found. The ther- 

 mal dilatation of the gasolene is found immediately from the difference 

 of the displacements of the two auxiliary experiments. It should be 

 noticed that the dilatation is used here in a sense slightly different 

 from the usual one in thermodynamics. The quantity given here is the 

 change in volume in cm.^ for 1° lowering of the temperature of the 

 quantity of gasolene which at that pressure occupies 1 cm.^. To find 

 the change of volume of the quantity of gasolene which originally at 

 0° and atmospheric pressure occupied 1 cm.^ we should need to know in 

 addition the compressibility of the gasolene. It is an advantage of the 

 method that it is not necessary to know this compressibility. It ir 

 now obvious why a knowledge of the compressibility of water at 0° is 

 necessary. The quantity of water is fixed. As the pressure is raised, 

 the part of the " first part of the volume " occupied by the water de- 

 creases in a way known, because the compressibility is known, and the 

 part occupied by the gasolene correspondingly increases. At any pres- 

 sure, therefore, the number of cm.^ of gasolene concerned in the total 

 dilatation of the " first part of the volume " is known, the dilatation 

 per cm.^, and so the total dilatation of the gasolene is also known, and 

 therefore the remainder of the total dilatation due to the water is 

 given immediately. 



There are several minor corrections to be considered. The change 

 of volume as given in cm.^ by the displacement of the piston involves 

 directly the diameter of the piston. This changes with pressure. 

 This correction has been already discussed in the mercury paper and 

 found to be 1.35 per cent for 10,000 kgm. The magnitude is almost 

 negligible for the present work, but the correction Wias nevertheless 

 applied in making the computations. Furthermore, the volume swept 

 out by the piston at constant pressure during a change of temperature 

 of the lower cylinder does not represent accurately the change of volume 



VOL. XLVII. — 29 



