As the traditions vary on the central fact, whether it was the bride who stabbed her husband, or the husband who stabbed the bride, no credence can be given to the mass of superstitions and spiteful slander which surrounded it, principally levelled at Lady Stair. Sir Walter Scott took the plot of his "Bride of Lammermoor" from this incident, but he disclaimed any intention of making Lord Stair the basis for Sir William Ashton. In 1670 Stair served as one of the Scottish commissioners who went to London to treat of the Union, but the project, not seriously pressed by Charles and his ministers, broke down through a claim on the part of the Scots to what was deemed an excessive representation in the British parliament. In January 1671 Stair was appointed Lord President of the Court of Session. In the following year, and again in 1673, he was returned to parliament for Wigtownshire, and took part in the important legislation of those years in the department of private law. During the bad time of Lauderdale's government Stair used his influence in the Privy Council and with Lauderdale to mitigate the severity of the orders passed against ecclesiastical offenders, but for the most part he abstained from attending a board whose policy he could not approve. In 1679 he went to London to defend the court against charges of partiality and injustice which had been made against it, and was thanked by his brethren for his success. When, in the following year, the Duke of York came to Scotland Stair distinguished himself by a bold speech, in which he congratulated the duke on his coming amongst a nation which was entirely Protestant. This speech can have been little relished, and the duke was henceforth his implacable enemy. His influence prevented Stair from being made Chancellor in 1681, on the death of the Duke of Rothes. The parliament of this year, in which Stair again sat, was memorable for two statutes, one in private law and the other in public law. The former, relating to the testing of deeds, was drawn by Stair, and is sometimes called by his name. The other was the infamous Test Act, probably the worst of the many measures devised at this period with the object of fettering the conscience by oaths. Stair also had a minor share in the form which this law finally took, but it was confined to the insertion of a definition of "the Protestant religion", by which he hoped to make the test harmless, but his expectation was disappointed.