Sir J. J. Thomson’s 1913 Rays Of Positive Electricity and 1921 Rays Of Positive Electricity Second Edition are online at the Internet Archive Million Book Project.
ChemTeam’s Classic Papers has J. J. Thomson On the Masses of the Ions in Gases at Low Pressures, Philosophical Magazine, 1899, 48, 547-567 and F. W. Aston Isotopes and Atomic Weights, Nature, 1920, 105, 617.
Some other classic mass spectrometry papers are available online: A. J. Dempster A new method of positive ray analysis Phys. Rev., 1918, 11, 316-324, A. G. Dempster, Positive-Ray Analysis of Potassium, Calcium and Zinc. Phys. Rev. 1922,20, 631-638, and J. A. Mattauch, J. A double-focusing mass spectrograph and the masses of N15 and O18 Phys. Rev. 1936, 50, 617-623 (subscription required).
Also see Scripps and Washington University for more references and links.
Here are two early papers that are referenced in Thomson’s book:
E. Goldstein, √É≈ìber enie noch nicht untersuchte Strahlungsform an der Kathode inducirter Entladungen. ”Berl. Ber. ”’1886”’ ”39” 691.
W. Wien, ”Verh. d. phys. Gesell.”, ”’1898”’, 17.
Here’s a recent paper on the history of canal rays:
Karl Wien, Brazilian Journal of Physics, vol. 29, no. 3, September, 1999, 100 Years of Ion Beams: Willy Wien’s Canal Rays (PDF)