BRIDGMAN. — WATER UNDER PRESSURE. 469 



^ = - 0.00722. 



dp 



It is to be noticed that this value is lower than the calculated value 

 usually given. This is because of the low value used for A F, which is 

 taken from the most recent work of Leduc.^ The effect of this change 

 is to bring the calculated value into better agreement with experimen- 



dr 

 tal values. Thus Dewar « found recently for -^, 0.0072. The slope 



at the origin as just calculated is shown by a dotted line in the diagram. 

 The agreement is within the limits of error. 



Even over this very low pressure range there are practically no other 

 results to compare these with. Tammann is the only other observer 

 who has gone to high enough pressures to find the direction of curva- 

 ture of the curve, which agrees with that found here. Dewar has 

 published results up to 250 kgm., and stated without publishing the 

 data that the curve remains linear up to 700 kgm. The departure from 

 linearity found above is about 6 per cent at 700. Several other ac- 

 counts have been published of the effect of pressure on the freezing of 

 ice, but the work has either been done only to a few atmospheres, or 

 else the work to high pressures has been only qualitative, like that of 

 Mousson.'' 



The second set of readings above gives the only determination of the 

 change of volume made in the present work. The procedure in deter- 

 mining four of these five points was the same as that used for nearly all 

 the other determinations with water as well as with mercury. This 

 consists in plotting piston displacement against pressure during de- 

 creasing pressure. This has the advantage at higher pressures of 

 almost entirely eliminating the effect of elastic after-effects in the con- 

 taining vessel, but this procedure is less essential at lower pressures. 

 For all ordinary substances this procedure means that the change of 

 volume is measured during melting and has the advantage that it 

 gives perfectly sharp values, it being impossible to overrun the melt- 

 ing curve. But here, due to the fact that ordinary ice is less dense 

 than the liquid, the method gives the change of volume during freez- 

 ing instead of melting, so that it is possible to considerably overshoot 

 the mark before solidification sets in. The change of volume is so 

 large during freezing, however, that this is no practical disadvantage, 

 as it is with substances showing a smaller change of volume. The 



6 Leduc, C. R., 142, 149-151 (1906). 



« Dewar, Proc. Roy. Soc. Lon., 30, 533-538 (1880). 



T Mousson, Pogg. Ann., 105, 161-174 (1858). 



