Although the author has not consulted unpublished material, it is necessary to draw the reader's attention to the following primary sources. The primary material for the study of Castlereagh's career is abundant, but scattered. Although Sir Charles Webster found the most important evidence for his two studies of Castlereagh's foreign policy in the Public Record Office in London, the value of his work was enhanced by his visits to the leading archives in Europe to study, in particular, the reports of foreign ambassadors from London. Similarly, Bradford Perkins, to give but one recent example, in his studies of Anglo-American relations down to 1822, has placed the student of Castlereagh in his debt by his researches in American archives. Consequently, although further research is unlikely to modify the overall interpretation of Castlereagh's foreign policy, many interesting sidelights may yet emerge from the archives of lesser states, both in Europe and elsewhere. One would like to know more about Castlereagh's relations with Spanish, Portuguese, Neapolitan and Spanish American diplomats, and intriguing figures such as Miranda and Pozzo di Borgo. The study of such evidence is likely to be rewarding in view of the emphasis which Castlereagh placed upon personal relations and contacts, on private explanations and gentle hints. To enumerate the possible sources for a comprehensive study of Castlereagh's foreign policy could therefore extend to a list of many national and state archives, and of the private papers of the leading statesmen and diplomats of the period.
In domestic affairs the situation is simplified geographically, but still raises extensive problems. In Irish matters, the Londonderry Papers, the Camden and Pelham Papers, and various other obvious sources have been thoroughly explored, but much of interest might well emerge from the private papers of Irish members of Parliament who were approached by Castlereagh and others during the preparations for the passage of the Act of Union. Similarly, it would be interesting to have more Catholic and Presbyterian accounts of Castlereagh's negotiations with them from 1798 to 1801. Equally