1. *Departamento de Fisica Teorica, Universidad Autonoma de Madrid, Cantoblanco, 28049, Madrid, Spain; josem.sanchez@uam.es.
2. I wish to thank Alexis De Greiff for his advice and valuable comments, and John Heilbron for his careful editing of the manuscript.
3. The following abbreviations are used: Anales, Sociedad Espanola de Fisica y Quimica, Anales; RAC, Rockefeller Archive Center; RMH, Revista Matematica Hispano-Americana.
4. HSPS, Volume 33, Part1, pages 3-31. ISSN 0890-9997. c2002 by The Regents of the University of California. All rights reserved. Send requests for permission to reprint to Rights and Permissions, University of California Press, 2000 Center St., Ste. 303, Berkeley, CA 94704-1223.
5. 1 BEFORE THE CIVIL WAR