Stamford takes Watson back to St. Bartholomew's where, in a laboratory, they find Holmes experimenting with a reagent, seeking a test to detect human hemoglobin. Holmes explains the significance of bloodstains as evidence in criminal trials. Watson raises their parallel quests to find a place to live. At Holmes's prompting, the two review their various shortcomings to make sure that they can live together. After seeing the rooms at 221B, they move in and grow accustomed to their new situation. Holmes reveals that he is a "consulting detective" and that his frequent guests are clients. Facing Watson's doubts about some of his claims, Holmes casually deduces to Watson that one visitor, a messenger from Scotland Yard, is also a retired Marine sergeant. When the man confirms this, Watson is astounded.