1U0 BRIDGMAN. 



Introduction. 



Our present knowledge of the compressibility of metals is mostly 

 due to the work of Richards l and his collaborators, and to Adams 

 Williamson and Johnston. 2 (I shall abbreviate reference to these 

 authors by A. W. J.) Richards has measured the compressibility of a 

 large number of metals over a pressure range of 500 kg. at room 

 temperature. A. W. J. have measured the compressibility of a smaller 

 number of metals over the much wider pressure range of 10000 kg/cm 2 , 

 but again only at room temperature. The work of Richards, there- 

 fore, does not enable us to find either the pressure or the temperature 

 variation of compressibility, and although the work of A. W. J. gives 

 valuable information as to the variation of compressibility with 

 pressure, they have themselves recognized that the pressure variations 

 so found are not accurate. 



Recent theoretical work, in particular that of Born 3 is now bringing 

 within the reach of the possibility of computation the compressibility 

 of substances in terms of their crystalline structure. Born's theory 

 of the compressibility of substances of the type of sodium chloride is 

 far enough advanced to give an expression for the variation of com- 

 pressibility with pressure. It seems therefore that the time is ripe for 

 a more careful experimental examination of the question of the com- 

 pressibility of the metals, although we may not have as yet a satis- 

 factory theory of the metallic state itself. 



The experimental work of this paper consists of a determination of 

 the compressibility of 30 metals over a pressure range of 12000 kg/cm 2 

 at 30° and 75°. This range is sufficient to give the pressure and 

 temperature coefficients of the compressibility. 



The question of experimental accuracy is an important one here. 

 It is well known that compressibility is one of the harder quantities to 

 measure experimentally, so that all the more is it difficult to measure 

 the pressure or the temperature coefficient of compressibility. The 

 use of high pressure is indispensable here, for by increasing the magni- 

 tude of the effects to be measured it is possible to attain the necessary 

 accuracy. This is particularly true with respect to the pressure 

 coefficient of compressibility ; the accuracy with which this may be 

 determined increases as the square of the pressure range. Other 

 things being equal, therefore, it should be possible with the pressure 

 range of this work to determine the pressure coefficient 570 times as 

 accurately as possible over a pressure range of 500 kg. 



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