England at this time was in close intercourse with Cromwell. Hyde, writing in January 1653–4, mentions the report that Digby had long held correspondence with Cromwell, and had done him good offices at Paris. In November 1655 a correspondent of Thurloe describes Digby as Cromwell's agent, and raises suspicions of his honesty. In letters dated February and March 1655–6 he is spoken of as Cromwell's confidant and pensioner. It seems certain that Digby thought to obtain from Cromwell full toleration for the catholics, and freely discussed the matter with him. In September 1655 a passport was granted him to leave England. In December he wrote to Thurloe in behalf of Calais merchants trading with England, and in March 1656, when complaining of the slanders of Sir Robert Welsh, expresses himself in full sympathy with Cromwell's government. At the time he was certainly engaged in diplomatic business on Cromwell's behalf, and was reported to be seeking to prevent an agreement between France and Spain. Digby's relations with Cromwell were warmly denounced by Clarendon in ‘A Letter from a true and lawful Member of Parliament’ in 1656, and by Prynne in his ‘True and Perfect Narrative,’ 1659, p. 41. In the summer of 1656 Digby was at Toulouse, and in 1658 lectured (according to his own account) at Montpellier on his ‘sympathetic powder.’ He afterwards visited Germany, but was in 1660 in Paris, whence he returned to England after the Restoration.
In spite of his compromising relations with Cromwell, Digby was well received by the royalists, and continued to hold the office of Queen Henrietta's chancellor. On 14 Jan. 1660–1 he received a payment of 1,325l. 6s. 8d. in consideration of his efforts to redeem captives in Algiers, apparently on his Scanderoon voyage. On 23 Jan. 1660–1 he lectured at Gresham College on the vegetation of plants. He was on the council of the Royal Society when first incorporated in 1663. In the following year he was forbidden the court. He gathered scientific men about him at his house in Covent Garden, and often ‘wrangled’ with Hobbes there. He died on 11 June 1665. The eulogistic elegy by Richard Ferrar is in error in stating that he died on his birthday. By his will dated 9 Jan. 1664–5 he directed that he should be buried at the side of his wife in Christ Church, Newgate, and that no mention of him should be made on the tomb. He gave all his lands in Herefordshire (lately purchased of the Duke of Buckingham), in Huntingdonshire, and on the continent to Charles Cornwallis, for the payment of his debts. His kinsman, George, earl of Bristol, received a burning-glass; his uncle, George Digby, a horse, and his sister a mourning-gown. His library was still in Paris, and was sold by the authorities for ten thousand crowns. The Earl of Bristol repurchased it.
Digby had five children, a daughter (Margery, married to Edward Dudley of Clopton, Northamptonshire) and four sons. Kenelm, the eldest, born 6 Oct. 1625, was killed at the battle of St. Neots while fighting under the Earl of Holland against Adrian Scrope, on 10 July 1648. John, born 19 Dec. 1627, married, first, Katherine, daughter of Henry, earl of Arundel; and secondly, Margaret, daughter of Sir Edward Longueville of Wolverton in Buckinghamshire, by whom he had two daughters. The elder daughter, Margaret Maria, married Sir John Conway of Bôdrhyddan, Flintshire, and her granddaughter, Honora, married Sir John Glynne. The children of Sir Stephen Glynne, Sir John's great-grandson, are the only living descendants of Sir Kenelm Digby. Sir Kenelm's two other sons (Everard, born 12 Jan. 1629–30, and George, 17 Jan. 1632–3) died young.
Digby's works in order of publication are as follows:—1. ‘A Conference with a Lady about Choice of Religion,’ Paris, 1638; London, 1654. 2. ‘Sir Kenelm Digby's Honour maintained’ (an account of the duel in France), London, 1641. 3. ‘Observations upon Religio Medici, occasionally written by Sir Kenelme Digby, Knt.,’ London, 1643, frequently reprinted in editions of Browne's ‘Religio Medici.’ 4. ‘Observations on the 22nd Stanza in the Ninth Canto of the Second Book of Spenser's “Faery Queene,”’ London, 1644. 5. ‘A Treatise of the Nature of Bodies,’ Paris, 1644; London, 1658, 1665, and 1669. 6. ‘A Treatise declaring the Operations and Nature of Man's Soul, out of which the Immortality of reasonable Souls is evinced,’ Paris, 1644; London, 1645, 1657, 1669. 7. ‘Institutionum Peripateticorum libri quinque cum Appendice Theologica de Origine Mundi,’ Paris, 1651, probably for the most part the work of Thomas White [q. v.] 8. ‘Letters between the Lord George Digby and Sir Kenelme Digby, Knight, concerning Religion,’ London, 1651. 9. ‘A Discourse concerning Infallibility in Religion, written by Sir Kenelme Digby to the Lord George Digby, eldest sonne of the Earle of Bristol,’ Paris, 1652. 10. ‘A Treatise of Adhering to God, written by Albert the Great, Bishop of Ratisbon, put into English by Sir Kenelme Digby, Kt.,’ Dec. 1653. Ded. to Digby's mother. 11. ‘A late Discourse made in a Solemne Assembly of Nobles and Learned Men at Montpellier in France, by Sir Kenelme Digby, Knight, &c. Touching the Cure of Wounds by the Powder of Sympathy. With Instructions how to make the said Powder.
