  A large sample of cosmic ray events collected by the CMS detector is
exploited to measure the specific energy loss of muons in the lead tungstate of
the electromagnetic calorimeter. The measurement spans a momentum range from 5
GeV/c to 1 TeV/c. The results are consistent with the expectations over the
entire range. The calorimeter energy scale, set with 120 GeV/c electrons, is
validated down to the sub-GeV region using energy deposits, of order 100 MeV,
associated with low-momentum muons. The muon critical energy in lead tungstate
is measured to be 160+5/-6 plus or minus 8 GeV, in agreement with expectations.
This is the first experimental determination of muon critical energy.
