BRIDGMAN. — MERCURY UNDER PRESSURE. 423 



cause of the diflFerence in compressibility between the solid and the 

 liquid mercury. The compressibility of mercury is small in any event, 

 and here the greater part of the piston displacement is used in com- 

 pressing the kerosene, so that only rough values can be expected from 

 the small difference. There is the further complication of the drift in 

 the temperature of the room. The slope does show a distinct ten- 

 dency to be less when the solid state is present, but the change is ir- 

 regular. It can be said, however, that the compressibility of the solid 

 at the freezing point is less than that of the liquid, the difference being 

 in the neighborhood of 10 per cent. 



The Change of Volume of Mercury o^ Freezing at 

 Atmospheric Pressure. 



It has already been stated that the necessity for redetermining the 

 change of volume at atmospheric pressure arose from the probable in- 

 accuracy of the previously accepted value (0.00260 cm.^ per gm.). The 

 inaccuracy of this number was first suspected when a set of values 

 found for Ay at different pressures, consistent among themselves to 

 1/25 per cent, did not extrapolate to within 2 per cent of 0.00260. 

 Investigation showed that the value 0.00260 is not a direct experi- 

 mental determination, but is obtained from the difference between the 

 density of the solid, as found by one observer, and the density of the 

 liquid as given by another. Since the change of volume is only about 

 3 per cent of the total volume, the density of both solid and liquid 

 must be known with an accuracy of sixty times the accuracy desired in 

 the change of volume. The following short discussion of these previous 

 experimental determinations will make clear that there are here possi- 

 bilities of error of fully the indicated magnitude. 



The value of the density of the solid seems to be open to by far the 

 most serious question. The commonly accepted value, in fact the only 

 value with any claim to accuracy at all, was given by Professor Mallet *5 

 of the University of Virginia. Tlie method consisted in freezing a 

 weighed quantity of mercury in a glass bulb, and then determining 

 the quantity of alcohol necessary to fill the bulb to a fixed mark. The 

 largest possibility of error seems to be in determining the volume of 

 the bulb at the freezing temperature of mercury. This was found from 

 the quantity of mercury filling it at O'' and 100°, using Regnault's 

 value for the dilatation of mercury, and extrapolating back to the 

 freezing point. No account at all was taken of the possibility of per- 



" Mallet, Proc. Roy. Soc. Lon., 26, 71 (1877). 



