  Recent simulations have predicted that near jamming for collections of
spherical particles, there will be a discontinuous increase in the mean contact
number, Z, at a critical volume fraction, phi_c. Above phi_c, Z and the
pressure, P are predicted to increase as power laws in phi-phi_c. In
experiments using photoelastic disks we corroborate a rapid increase in Z at
phi_c and power-law behavior above phi_c for Z and P. Specifically we find
power-law increase as a function of phi-phi_c for Z-Z_c with an exponent beta
around 0.5, and for P with an exponent psi around 1.1. These exponents are in
good agreement with simulations. We also find reasonable agreement with a
recent mean-field theory for frictionless particles.
