  We present measurements of drift velocities and gains in gas mixtures based
on Ar and Xe, with CO2, CH4, and N2 as quenchers, and compare them with
calculations. In particular, we show the dependence of Ar- and Xe-CO2 drift
velocities and gains on the amount of nitrogen contamination in the gas, which
in real experiments may build up through leaks. A quantification of the Penning
mechanism which contributes to the Townsend coefficients of a given gas mixture
is proposed.
