Lecturer in Natural Philosophy a n d M a r j o r ie B. M 'E w e n , B.Sc., Demonstrator in Physics, The University, St Andrews {Communicated by H. S. Allen, F.R.S.-Received 11 March 1938) I n t r o d u c t io n The occurrence of band spectra in the chemically monatomic gases leads to the belief that molecules are formed in the gas by the electrons or photons producing the spectra. Arnot and Milligan (1936) and Arnot and M'Ewen (1938) have shown that diatomic mercury molecules are formed by the

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... ttachment of excited atoms in P-states to each other and to normal atoms. Although it has been inferred from the band spectra found in helium that molecules are formed by some such attachment process, these have not previously been studied electrically, nor have the details of the attachment process been investigated.* No definite evidence has been put forward to show that band spectra occur in the other rare gases. This paper contains the results of an investigation made by the balanced space-charge method of the ionization produced in argon, neon and helium when these gases are subjected to bombardment by electrons whose energy was varied from a few volts below the first excitation potential to a few * In th e early experim ents on critical potentials ions were found to be produced in helium w hen th e bom barding electrons had energies less th a n th a t corresponding to th e atom ic ionization potential. These effects were generally a ttrib u te d to im purities in th e helium used, b u t ap parently never to the form ation of helium m olecules.