  We measure the spin lattice relaxation of the In(1) nuclei in the CeMIn_5
materials, extract quantitative information about the low energy spin dynamics
of the lattice of Ce moments in both CeRhIn_5 and CeCoIn_5, and identify a
crossover in the normal state. Above a temperature T* the Ce lattice exhibits
"Kondo gas" behavior characterized by local fluctuations of independently
screened moments; below T* both systems exhibit a "Kondo liquid" regime in
which interactions between the local moments contribute to the spin dynamics.
Both the antiferromagnetic and superconducting ground states in these systems
emerge from the "Kondo liquid" regime. Our analysis provides strong evidence
for quantum criticality in CeCoIn_5.
