We can see a physical reason here for choosing the separation constant to be
negative. As t increases, the temperature of a body might decrease to zero as in
(3.6), but it could not increase to infinity as it would if we had used +k2 in (3.5)
and (3.6). The space equation in (3.5) is the Helmholtz equation (1.5) as promised.
You will find (Problem 10) that the space part of the wave equation is also the
Helmholtz equation.