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Assuming now th a t the mean energy loss due to ionization is constant, i.e. of coincidence experiments on cosmic rays 521 which is correct down to energies of the order of 100,000 eV, our Plon will be a (^-function Pi0Xl{E",l")AE" = A{E" ^E C)8{E"-ft") A E", (5) Vol. CLXVIII. A. X This shower u n it length has th e value 0-358 cm. for P b, 0-215 cm. for Au, 0-207 cm. for P t, 0-824 cm. for Sn, 1-02 cm. for Cu, 1-26 cm. for F e, 6-71 cm. for A1 an d 15-5 cm. for C in form of g rap h ite, d ensity 2-3 g./cm .3. (The value 7-8 for A1 as given b y B h ab h a an d H eitler was incorrect.

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