BRIDGMAN. — MERCURY UNDER PRESSURE. 359 



seasoning by subjecting to pressure several times before the series of 

 measurements is begun. One at least of the previous applications 

 should be to a pressure as high if "not higher than that to be reached 

 during the measurements. After treatment like this the piezometers 

 show no further change of volume, and tbe results with any one are 

 fairly regular. It is a question, however, how good a criterion uniform- 

 ity of results and permanency of volume are for the uniform comi^ression 

 of the piezometer without change of shape. It must be confessed there 

 seems no valid reason why the piezometer after proper seasoning 

 should not settle down into a steady state in which it gives consis- 

 tent results with change of shape, and probably it does. The difference 

 of the results with different piezometers is probably to be explained by 

 this effect. The best that can apparently be done is to take the mean 

 of the determinations with several piezometers, all of which individually 

 give consistent results. All of these considerations apply chiefly to 

 determining the compressibility of the incompressible mercury ; they 

 have much less weight, and indeed are almost negligible when applied 

 to water. 



There is one particular way in which it is very easy to produce 

 irregular results by strains in the piezometer, and one which caused 

 much trouble before the correct explanation was found. If the pressure 

 is pushed too high, the water freezes with sudden decrease of volume. 

 This was not expected when entering on this work, since the freezing 

 curve of water as found by Tammann^^ gives no evidence of the 

 existence of ice above zero. The fact is that there are other forms of 

 ice besides those found by Tammann, one of which is stable at room 

 temperatures. This will be treated in a following paper. The sudden 

 decrease of volume during freezing is accompanied by a rush of liquid 

 through the narrow channel, and, while this rush is taking place, by a 

 momentary excess of pressure outside over that inside. This excess 

 pressure may produce volume set, or if not sufficiently great for that, 

 at least set up internal strains which produce irregularities on subse- 

 (juent determinations. The irregularities so introduced may be com- 

 paratively very high; as much as 10 to 15 per cent in the case of 

 mercury on the next determination. The next value is nearly always 

 too low. The irregularities become smaller with subsequent determin- 

 ations, but the only satisfactory way is to anneal the piezometer again. 

 Many of the early data had to be discarded because of this freezing 

 effect. 



As a consequence of the unforeseen freezing of water at temperatures 



" Tammann, loc. cit., p. .315-.344. 



