  The 1st law of thermodynamics for heat exchange is dQ=dU+PdV. According to K.
Martinas etc., J. Non-Equil. Thermod. 23 (4), 351-375 (1988), for substances
with negative thermal expansion coefficient, P in this law is negative. In the
present paper it has been shown that P for such substances is positive but the
sign before P must be minus not plus: dQ=dU-PdV.
