  The magnetic entropy of YbRh_2(Si_{0.95}Ge_{0.05})_2 is derived from
low-temperature ($T\geq 18$ mK) specific heat measurements. Upon field-tuning
the system to its antiferromagnetic quantum critical point unique temperature
over magnetic field scaling is observed indicating the disintegration of heavy
quasiparticles. The field dependence of the entropy equals the temperature
dependence of the dc-magnetization as expected from the Maxwell relation. This
proves that the quantum-critical fluctuations affect the thermal and magnetic
properties in a consistent way.
