Results from recent high pressure experiments have shown ΔT in the relationship (&agr;/&agr;0)=(&rgr;/&rgr;0)-ΔT to be constant over significant compression, one of the original assumptions required to make this relationship valid. We have re-examined the original derivation of this equation and present a simpler derivation eliminating the previous assumptions. Furthermore, high pressure experiments find (∂ln&agr;/∂ln&rgr;)T is equal in value to 1/&agr;KT(∂KT/∂T)P derived from experiments at ambient pressures. From values of -ΔT=1/&agr;KT/∂T)P at high temperatures, it is asserted that -(∂ln&agr;/∂ln&rgr;)T must lie between 4 and 6 in the lower mantle, a value significantly higher than previously assumed in earth models. ¿ American Geophysical Union 1990