Utterson visits Jekyll to which he is extremely unwell - compared to being "deathly sick".
Utterson begins to question Jekyll about his relationship with Hyde and if Jekyll had been blackmailed to put him in his will. Jekyll confirms and also shows Utterson a letter from Hyde which says he will never hear from Hyde again - Utterson takes the letter and leaves.
Despite what Jekyll had told Utterson, he was still suspicious and therefore took the letter to Mr Guest who was Utterson's clerk
Mr Guest was an expert in handwriting and identified that there were similarities between Jekyll's writing and Hyde's - he speaks of how this was concealment