402 PROCEEDINGS OF THE AMERICAN ACADEMY. 



friction of the packing prevented all motion after the initial increase 

 of pressure. 



The above procedure, reading of pressure and piston position, was 

 repeated at appropriate pressure intervals up to several hundred or 

 perhaps a thousand kgm. beyond the freezing pressure. On plotting 

 piston displacement against pressure, the freezing is shown by a dis- 

 continuity in the curve. Similar readings were also made on release 

 of pressure, and so another value for the change of volume found. On 

 releasing pressure the discontinuity is more sudden than with increas- 

 ing pressure, because it is impossible to superheat the crystalline solid 

 on decreasing pressure, but the liquid may be subcooled on increasing 

 pressure. Because of this, and for another reason to be explained 

 later connected with the distortion of the steel, the value found with 

 decreasing pressure was without exception accepted as the correct value. 



In the use of this apparatus and in the calculation of the data, there 

 are numerous corrections or disturbing factors which will now be 

 considered in detail. 



Leak. — As already stated, the most obvious and apparently the 

 greatest difficulty with the method, particularly at high pressures, is 

 the question of leak. It might perhaps be possible to correct for this 

 as was done sometimes by Tammann by weighing the quantity of liquid 

 escaping past the piston in a given time, but this correction would always 

 be unsatisfactory in cases where, as here, the total motion of the piston 

 during freezing is small (0.2 inch) and many extend over an hour or 

 more. It has been stated that there was absolutely no leak. This was 

 proved by the fact that the piston and upper end of the packing, when 

 removed from the cylinder after the end of an experiment, were always 

 perfectly dry, even though the total time over which pressure has been 

 applied might reach into several days, as it did on several occasions. 

 That there was no leak in any other part of the apparatus was proved 

 by the self-consistency of the results as well as by special experiment. 

 On one occasion pressure was left at 7500 over night, temperature 

 being kept constant with a bath of ice and water, and in the morning 

 had not dropped by as much as the smallest amount perceptible with 

 the manganin resistance, namely 2 kgm./cm.^ 



The effect of leak, if there were one, would be to make the values of 

 Ay during increase larger, and during decrease smaller. The values 

 did actually in the great majority of cases disagree in this way. That 

 this was not due to leak will be evident from a discussion of 



Elastic After-Effects. — In the ordinary sense of the word, elastic 

 after-effects, as is indicated by the name, are purely elastic phenom- 

 ena, complete recovery after any application of stress always occurring, 



