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The number of citations per year for three highly cited physics papers from the pre-1900 and pre-1930 eras. Ewald's 1921 paper has always been highly cited (green line; 1921 Ann. Phys.,Lpz 64 253), but citations to the papers by Kohlrausch (red; 1847 1921 Ann. Phys.,Lpz 12 393) and von Smoluchowski (blue; 1917 Z. Phys. Chem. 92 129) only became significant when they were cited in other papers in the mid-1980s.