For instance, during a debate held on 27 July 1891 in the House of Commons of the United Kingdom, one member, Mr Atkinson, said the following:
Mr. Speaker, Sir, may I very respectfully claim freedom of speech in order to call your attention to the Journals put before the House to-day, which contain a statement with reference to me which, if it were true, would make me ashamed of myself and my conduct as a Member of Parliament for the remainder of my days.
Another member, Mr Goschen, said:
I am only expressing the feelings which obtain universally in every part of the House when I say that you, Sir, have shown courtesy and impartiality to all sides of the House and to every Member of the House. You, Sir, have been obliged to appeal to the House against a Member of this House and you have said it is intolerable that during the whole of last week you have had to complain of the conduct of the hon. Member. I venture to think it will be the universal feeling of the House that the protection which you have asked at our hands must be accorded to you, Sir, unanimously. Without wishing to bear hardly on the hon. Member, without wishing in any way to show towards him any animosity, I still think the House will be of opinion that what you have said, Sir, must be marked, and must be marked in such a manner that it may be known that you have the support of the House.
What part of speech does, "Sir", the restated form of address, constitute and what is its purpose?