Nissen, who came to Vienna, after Mozart's death, as a Danish diplomatist, became acquainted with his widow, and interested himself in her unprotected condition. He had a great turn for business matters, and was fond of arranging papers, writing letters, and even copying, without understanding what it was that he was occupied about. He therefore willingly undertook to put Mozart's effects in order, to assist the widow in all her business arrangements, and to carry on her correspondence. A long series of letters which he wrote in her name show him to have been a well-meaning, sensible man, somewhat over-circumstantial in his style of writing. After his marriage with Mozart's widow he felt it his duty to labour with the same conscientious care for his memory as he had formerly done for his property, and he employed the leisure of his remaining years, which were spent at Salzburg, in carrying out this design.
We ought to own ourselves deeply indebted to him, for without his care the most important documents and traditions would have been hopelessly lost. Mozart's sister was then living at Salzburg; her recollections, and those of his wife, afforded an abundance of characteristic traits, and the carefully preserved papers and family correspondence, were a rich mine of authentic documents.
Besides a number of separate deeds, letters, and memoranda, he had at his disposal: Leopold Mozart's letters to Hagenauer during the journey to Vienna (September, 1762, to January, 1763); during the great journey (from June, 1763, to November, 1766); during the Vienna journey (September, 1767, to December, 1768); letters both of the father and son to their family during the Italian journey (December, 1769, to March, 1771; from August 13, 1771, to December, 1771; from October, 1772, to March, 1773); from Vienna (July, 1773, to September, 1773); from Munich (December, 1774, to March, 1775); Wolfgang's and his mother's letters home, together with the answers of Leopold and his daughter during the journey to Paris (September, 1777, to January, 1779); Wolfgang's correspondence with his father and sister during his journey to Munich and residence in Vienna. Wolfgang's letters come down to 1784, his father's to 1781.