  We emphasize that the pressure related work appearing in a general
relativistic first law of thermodynamics should involve {\em proper volume
element} rather than coordinate volume element. This point is highlighted by
considering both local energy momentum conservation equation as well as
particle number conservation equation. It is also emphasized that we are
considering here a {\em non-singular} fluid governed by purely classical
general relativity. Therefore, we are not considering here any semi-classical
or quantum gravity which apparently suggests thermodynamical properties even
for a (singular) black hole. Having made such a clarification, we formulate a
global first law of thermodynamics for an adiabatically evolving spherical
perfect fluid. It may be verified that such a global first law of
thermodynamics, {\em for a non-singular fluid}, has not been formulated
earlier.
