BRIDGMAN. — MERCURY UNDER PRESSURE. 405 



the following one, the discrepancy was reversed in sign, the values 

 during freezing being too low, while those during melting still lay on 

 the same smooth curve as before. This certainly cannot be due to 

 leak. Neither is it in accord with the general nature of other elastic 

 after-effects at low pressures. But other experience of this same general 

 nature at these high pressures indicates that this is probably the gen- 

 eral nature of the effect at high pressures. It is as if the subsequent 

 application of pressure, and possibly the change of temperature also, 

 changes somewhat the position of the molecules, like shaking a magnet, 

 so that they become free again to resume their natural elastic recov- 

 ery from the previous greater maximum. One experiment with a hol- 

 low steel cylinder subjected to external hydrostatic pressure strikingly 

 suggested the same general explanation. This cylinder had been pre- 

 viously subjected to a pressure many times beyond the elastic limit, so 

 that the metal had flowed toward the center, decreasing the internal 

 volume. On removing the pressure there was some slight elastic re- 

 covery of the internal volume, but not nearly as much as was to be 

 expected. But the next increase of external pressure produced in the 

 initial stages a very marked increase of internal volume, instead of 

 a decrease, the natural effect of the pressure. It must be that in some 

 way the pressure loosened up the molecules so that they were free to 

 resume their natural recovery. 



Other slight differences suggest also that these discrepancies are of 

 the general nature of elastic after-effects. Because of breakage, several 

 different pieces of apparatus were used. The discrepancy was always 

 greatest with the thickest-walled vessels. It has been observed sev- 

 eral times in other connections that elastic after-effects are greater with 

 the greater mass of metal, because this allows greater heterogeneity of 

 strain and greater volume in which to store the effect, with consequent 

 longer time of recovery. At a given pressure the effect in a new piece 

 of apparatus was smaller before it had been subjected to the greatest 

 maximum than it was at this same pressure after being so subjected to 

 the maximum. In general, the effect was likely to increase with time, 

 showing fatigue of the metal. In one case, in the similar measure- 

 ments on water, to be described elsewhere, rupture of the vessel was 

 foreshadowed by an increase of the effect beyond 3 per cent. 



There may be some slight effect of permanent set mixed up with 

 the above effects. An undoubted set was always very distinctly ob- 

 vious after the first application of the greatest maximum. This appli- 

 cation was usually made in a preliminary test in order to season the 

 apparatus before the measurements were undertaken. This applies 

 particularly to those three or four occasions after the lower cylinder 



