1. J. B. Anderson, R. P. Andres, and J. B. Fenn, inAdvances in Chemical Physics, edited by John Ross (Wiley, New York, 1966), Vol. X, p. 275.
2. U. Bossel, F. C. Hurlbut, and F. S. Sherman,Rarefied Gas Dynamics, edited by L. Trilling and H. Y. Wachman (Academic, New York, 1969), Vol. II, p. 945.
3. We used type 303 stainless steel, but this is probably not essential.