Hall acquired a reputation as a geologist, having early developed an interest in that science; he studied rock formations in Scotland, then in the Alps and Italy, returning to England in 1785. He wrote memoirs on the subject for the Royal Society of Edinburgh, of which he became president, and also a curious Essay on Gothic Architecture (1797).
In May 1806 Hall was put up by his brother-in-law Thomas, 5th Earl of Selkirk (a Whig who was trying to persuade Lord Grenville to make him a representative peer of Scotland) for Kirkcudbrightshire, in opposition to Lord Galloway’s interest. The indignant patron described Hall as ‘a declared democrat and an avowed atheist, but clever’, and was doing his best to frustrate Hall’s candidature when he was forestalled by the dissolution, which came before Hall had been on the roll of freeholders for a year and disqualified him from standing.
Hall voted against the convention of Cintra, 21 Feb. 1809. In the debate on the Duke of York’s misconduct of army patronage, 17 Mar., on which he had cross-examined witnesses several times the month before, he began to comment on evidence, ‘when symptoms of impatience appearing in the House, the worthy baronet good-humouredly observed that he could take a hint as readily as any one’ and contented himself with remarking that there was insufficient evidence to give a verdict, so he would oppose Perceval’s motion, though he had supported Turton’s that day; and he subsequently (15 June) claimed that he had supported Wardle’s address of 15 Mar. critical of the Duke’s conduct, if this was what he meant by saying he ‘was one of a minority of 125’ on the subject ‘though his name had not appeared in the published lists’. On 15 June he was criticizing Burdett’s plan of parliamentary reform as an inducement to radical revolution and defended the existing order: ‘even the rotten boroughs might ultimately do a great deal of good. He had got into better company than he at one time imagined ... [the House’s] votes were almost always satisfactory to the nation’. Three days before he had been in the minority who objected to the title of Curwen’s reform bill.
Hall voted against the address, 23 Jan. 1810, and for the motions against the Scheldt expedition, 26 Jan.-30 Mar. 1810: lamenting its failure, 27 Mar., he said a partial change of administration was needed and he had no objection to the Marquess Wellesley: he had, in a speech of 17 June 1808, looked forward to Wellesley’s playing a role in the government. He was listed by the Whigs as one of the present opposition at that time. On 27 Mar. and 10 Apr. he was on the side of caution in the debates on Sir Francis Burdett’s conduct. On 16 Apr. he spoke and voted in favour of releasing from Newgate John Gale Jones, ‘a sort of trading orator’: he was subsequently less indulgent to Burdett and spoke of his ‘idle and boyish mutiny’, 7 June. Next day he voted for Williams Wynn’s motion in defence of the privileges of the House. His own motion for the discharge of Gale Jones was unsuccessful, 15 June. He had been in the majority against parliamentary reform on 21 May.
There is no further indication of parliamentary activity. On 22 Dec. 1810 Hall was granted leave of absence for illness and Lord Lauderdale, writing to Lord Grenville on 14 Aug. 1811, reported that he had been ‘unwell and could not attend Parliament’ last winter, but had authorized him to announce to Lords Grenville and Grey ‘that he meant in future uniformly to support you in Parliament’. Lauderdale regarded this as a reason for doing something for Hall’s son Basil in the navy, who wished to accompany Sir Samuel Hood to India.
