550 Mr. W. Sutherland on a 



applied longitudinally to a cylinder of length L producing a 

 change of length dL, then 



q=FL/dL (a). 



He also observed directly the change of volume of the 

 cylinder in this experiment, but 



dV=(l-2er)VP/2 (b); 



hence these two experiments give a direct determination of o. 

 Again, he compressed the hollow cylinder of radii P and R 

 externally with hydrostatic pressure, and according to elastic 

 theory, if the material of the cylinder is isotropic the change 

 of volume is given by 



dV = (5-4o-)R 2 PV/(R 2 -B. 2 )2 0). 



Amagat used this equation and (b) to calculate a and q from 

 his data ; but a somewhat more trustworthy method would be 

 to substitute the directly observed value of q in (c) and so ob- 

 tain cr for comparison with the direct value given by (a) and 

 (b) . To complete the available evidence for the isotropy of 

 the metals and alloys I will reproduce Amagat's directly 

 found values of o- and o- 1? his values got from (b) and (c) as 

 cr 2 , and values which I have calculated from (a) and (c) as <r 3 . 

 Steel. Copper. Brass. Delta. Lead. 



oi -268 -325 -324 -347 -431 



o- 2 -269 -329 -330 -333 -425 



0-3 -263 -303 -301 -388 -493 



The agreement is sufficient to justify the assumption ot 

 isotropy on which equation (c) is founded. 



9. Dulong and Petit's Law, Joule and Kopp's Law, and the 

 Equation for Compound Solids. — The physical meaning of 

 Dulong and Petit's law of the constancy of the atomic heats 

 of the elements has been a subject of speculation with chemists 

 and physicists ever since its discovery. As the kinetic theory 

 of gases in regarding temperature as proportional to kinetic 

 energy of the molecule, required that apart from internal 

 motion and work the molecular heats of all gases should be 

 the same, it has been felt that a similar connexion between 

 temperature and molecular kinetic energy is at the founda- 

 tion of Dulong and Petit's law. The kinetic theory of solids 

 makes this quite plain, that Dulong and Petit's law is simply 

 the expression of the fact that temperature is proportional to 

 the kinetic energy of the molecule, or, to be more accurate, of 

 the atom. The meaning of Dulong and Petit's law becomes 



