COMPRESSIBILITY OF METALS. 241 



Summary. 



A new method has been developed by which the linear compressi- 

 bility of metals or other solid substances with small compressibility 

 may be measured to high pressures. The method is accurate enough 

 to give change of compressibility with pressure and temperature. 

 This paper contains the results of measurements by this method on 

 30 metals over a pressure range of 12000 kg/cm 2 , and at 30° and 75°. 

 The majority of these metals crystallize in the cubic system, or else in 

 the hexagonal close packed arrangement of spheres, and for them the 

 change of volume may be obtained immediately from the change of 

 linear dimensions, because the compressibility is the same in all direc- 

 tions. Formulas are given for the change of volume as a function of 

 pressure and temperature over the range of the measurements. In 

 general the compressibility decreases with increasing pressure and 

 increases with increasing temperature. The order of magnitude of 

 the change of compressibility or thermal expansion with pressure is 

 the same for all metals. The compressibility changes under pressure 

 by a fraction which is a small number (varying from 2 to 30) times the 

 proportional change of volume under the same pressure, and similarly 

 the proportional change of thermal expansion under pressure is of the 

 order of a small number times the corresponding proportional change 

 of volume. 



Six of the thirty substances do not crystallize cubic or in the hex- 

 agonal close packed arrangement of spheres, and for these it was 

 established that there are large differences of compressibility in differ- 

 ent directions; for one substance, tellurium, it was found that in one 

 direction the linear compressibility is even negative. 



In the theoretical discussion it was shown that it is very probable 

 that the forces resisting compression in a metal are the same in nature 

 as those in a salt of the type of NaCl, that is, the metal may be re- 

 garded as a lattice of ions and electrons acting on each other by electro- 

 static forces due to one or more single elementary charges. In addi- 

 tion there is a force of repulsion. Criticism is made of the details of 

 Born's proof of the inverse ninth power law for the potential of the 

 repulsion, and the conclusion drawn that we have not at present a 

 detailed enough knowledge of the structure of the atom to determine 

 the law of repulsion so accurately as to allow us to differentiate the 

 formulas once and twice, as is necessary in computing the compressi- 



